Taking this from "A Guide to the Star Wars Universe" by Bill Slavicsek...2nd edition...page 47. (This book was written after the author spoke with two ladies from Lucasfilm. I know this because he acknowledges them at the beginning of the book.)
"When Kenobi saw what had become of Anakin, he tried to disuade him and draw him back from the dark side.
According the the ROTJ novel, discussion turned to battle, and the two fought fiercely. It ended when the man who had been Anakin Skywalker fell into a pit of molten lava."
Its right there and straightforward.