Tai Altman, from Auldearn, near Inverness, said he knew that his parents, Raphael, 69, and Tamar, 72, were planning to take their own lives.
The couple, both supporters of right-to-die group Dignity in Dying, had marked the date 3 October, 2013, in their diary with the single word “depart”.
The coroner at the inquest in Oxford was told that the couple, who were married for 46 years, had decided to end their lives together in a “dignified” way.
They drank a liquid barbiturate and were found dead in each other’s arms on their bed.
Their son said he knew the devoted couple would choose to die together if they could and, with his father’s cancer reaching its final stages, he realised the time had come.
The father-of-three said his parents took a lethal potion provided by Dignitas – the Swiss organisation which helps people end their lives. He said his parents had turned down the lights and put on soothing music before pouring a liquid barbiturate into a glass and a bowl. “We spent two days saying goodbye. We got the chance to say everything we wanted to say,” said Mr Altman. “I told them they had been good parents and I felt honoured to have had them and the values they had given me would be passed to their grandchildren.
“I wanted to give them a feeling that they would leave something tangible behind.”
His father, a former religious education teacher, was diagnosed with Hodgkin’s lymphoma in 2006 but he enjoyed a good quality of life until a few days before he died.
Mrs Altman’s health was poor. She suffered from diabetes, neuralgia and muscular problems, and made it clear to her family she was not prepared to live without her husband. “On more than one occasion I gave my mother a scenario in which she could stay,” he said. “I told her she could move here and be with us and her grandchildren, but she was not without her own physical ailments. She wasn’t about to relocate 500 miles. I didn’t argue about it.”
He spent two days with his parents at their home in Oxford in October and knew he would never see them alive again.
“When my father lost the motor control of his body, I knew it was close to the end,” he said. “I did a lot of crying because I knew what was going to happen.
“We got him out of hospital on the Monday. He couldn’t walk, he couldn’t even feed himself. A few days earlier, he was able to get up and down the stairs and take care of himself. It was good that it happened so quickly. Who would want to spend months and months in pain?”
The family went back to Mr Altman’s parents’ home and spent the next two days together. “I had already asked them how they were going to do it,” he said. “They said it was the same stuff that Dignitas uses, which is a chemical compound. I didn’t have any questions.”
Mr Altman received an e-mail from his father, as planned, telling him to contact their family doctor.
The doctor found the couple dead in their bed at their home in Great Milton, near Wheatley. Mrs Altman’s head was resting on her husband’s shoulder, her arm over his chest.
“I always knew they were mated for life,” said Mr Altman, 43, a driving instructor.
“They were hippies – dad was from Cape Town, mum was a New Yorker. Before they moved to England to raise us, they set up a kibbutz, they lived on a Spanish island with other hippies with no running water or electricity.
“They were free-thinking people with their own mindset,” he added.
Mr Altman believes the suicide potion was purchased “surreptitiously” over the internet. “I played no part in that,” he said. “They were big campaigners of the right to die. My father gave evidence to a government commission on assisted dying. I understand their beliefs and I am happy to support the right to die.”
The coroner recorded verdicts of suicide. He said: “It is clear they intended to die together as they lived together.”