Vera Kriegel and her twin sister Olga were just five years old when they were taken from their village in Czechoslovakia to Auschwitz.
Transported in cattle cars which were so tightly packed that the dead were still standing, she recalls the "sheer terror" of arriving at the camp and treading on "dead people like steps" as she left the train.
New arrivals at the camp were sorted into the weak, who would be gassed straight away, and the strong, who would be made to work. But Mengele and his assistants were there too, looking for twins.
Vera, her sister, and her mother were taken straight to SS Captain Josef Mengele. He was intrigued, she says, by what he described as her mother's "perfect Aryan features" and blue eyes, while Vera's and her sister's were brown.
Mengele selected them for experimentation.
Another woman who remembers her arrival at the camp is Jona Laks, who was taken as a teenager from the Lodz ghetto. She was not immediately recognised as a twin and was initially sent off in the direction of the gas chamber - when her sister told Mengele they were twins he had her brought to his laboratory.
Josef Mengele was an assistant to a well-known researcher who studied twins at the Institute for Heredity Biology and Racial Hygiene in Frankfurt - he started working at Auschwitz in May 1943.
There he had an unlimited supply of twins to study, and he wouldn't get in trouble if they died.
According to Prof Paul Weindling of Oxford Brookes University, author of Victims and Survivors of Nazi Human Experiments, hundreds of children were used in Mengele's experiments.
"I found a record of a prisoner doctor and bacteriologist who was forced to work for Mengele that there were 732 pairs of twins," he says, and suggests the doctor was interested in genetics. "I think Mengele might have been interested in the inheritance of the propensity to having twins."
He believes many of the twins survived Auschwitz, although he thinks Roma twins were almost certainly killed.
Some of the children, now elderly, have little memory of the experiments, others have memories that may not be 100% accurate.
Jona Laks says Mengele removed organs from people without anaesthetic, and if one twin died the other would be murdered. Vera Kriegel says that he killed people with an injection to the heart, and then dissected them.
Find out more
Watch more about how Josef Mengele experimented on twins at Auschwitz on Newsnight or on BBC World News at 23:30 GMT on Friday 30 January.
She remembers being ushered into his laboratory. "I was looking at a whole wall of human eyes. A wall of blue eyes, brown eyes, green eyes. These eyes they were staring at me like a collection of butterflies and I fell down on the floor."
The first experiment she was subjected to involved being kept in a small wooden cage with her sister and being given painful injections in her back - she doesn't know why, but thinks it may have been an attempt to change the colour of her eyes.
In another experiment, she says, the pair of them and more than 100 other twins were given injections of bacteria that cause Noma disease - an infection of the mouth or genitals, which causes boils and often turns gangrenous.
Some twins became feverish, and some died, she says. She also remembers Mengele reacting angrily when twins went missing - once when this had happened she stared him out to prove he could not completely dominate her.