Prosecutors determined that the case against Alan Purdy couldn't be proven beyond a reasonable doubt, said Tanya Sierra, a spokeswoman for the district attorney.
The office routinely declines to explain why charges won't be filed in cases they decide not to pursue, Sierra said.
Margaret Purdy, 84, was found dead with a bag over her head in March, and her death was ruled a suicide by the county medical examiner. Family said the woman battled a series of ailments as her husband doted on her in her final years, and had attempted suicide before her death.
Family members were opposed to the criminal charges against Alan Purdy, who was released on $15,000 bail after the arrest.
His sister-in-law Margot Smith told The Associated Press Wednesday that it would have been awful if prosecutors had decided to pursue a case.
"I'm absolutely delighted to hear it. He's 88 years old and hard of hearing and he loved his wife dearly," said Smith.
Smith added that Alan Purdy was so hard of hearing that he had trouble making out what authorities were saying to him at the time of his arrest.
"I'm delighted to hear this," Purdy's daughter Catherine Purdy, a Berkeley psychologist, told the Los Angeles Times Wednesday. "I feel like justice has finally happened."
The Times reports that the once vibrant woman left a suicide note on her desk after being bedridden in her final years from severe pancreatitis, an autoimmune disease, a crumbling spine and three fractured vertebrae that never healed.
At the time of the arrest, the couple's son-in-law John Muster, told The Associated Press that it wasn't the first time Purdy's wife had tried to commit suicide.
"She had mentioned for some time that she was under a great deal of pain and that this was a very hard life," Muster said in a telephone interview from Berkeley.
The Purdys were close friends for many years and proved a perfect match when they married later in life, relatives said. Margaret Purdy kept a close eye on her husband, who lost much of his hearing with age. He, in turn, watched after her as she battled a series of ailments in recent years.
Alan Purdy, a pilot with a doctorate in biomedical engineering, worked for years at the federal Occupational Safety and Health Administration, and Muster said both were "fully functioning mentally."
There is no specific federal law regarding either euthanasia or assisted suicide. All 50 states and the District of Columbia prohibit euthanasia — which is when a doctor actively kills a patient — under general homicide laws.
California is one of three dozen states that have specific laws prohibiting assisted suicides. Seven ban assisted suicide under common law.