"His contribution to HIV research and treatment, and his determination to ensure access to those treatments for people in Africa and Asia cannot be underestimated," said David Cooper, director of the Kirby Centre at Australia's University of New South Wales.
The 59-year-old Dutchman was on his way to Melbourne for the 20th International AIDS Conference, which starts on Sunday.
His partner Jacqueline van Tongeren and dozens of other conference delegates were also among the 298 people killed when the jet was downed over conflict-torn eastern Ukraine.
"I can confirm Joep Lange was on the plane," Andrea de Graaf, spokeswoman for the PharmAccess Foundation founded by Lange in 2000 to boost access to HIV drugs in Africa, told AFP.
Colleagues described Lange, a former president of the International AIDS Society (IAS) that organises the biennial AIDS conference, as a dedicated researcher and activist for accessible treatment, as well as a lover of books.
"My good friend Prof. Joep Lange was one of the most creative AIDS researchers, a humanist, and tireless organiser, dedicated to his patients and to defeating AIDS in the poorest countries," said former UNAIDS executive director and London School of Hygiene and Tropical Medicine head Peter Piot.
"Global health and the AIDS response have lost one of their great leaders."
Lange began his HIV/AIDS research in 1983, when it was still a new, ill-known disease.
Jaap Goudsmit, a PharmAccess co-worker and friend, said Lange had witnessed the first AIDS cases in Holland as a physician.
"He was very moved by such young people getting this very crazy disease which killed them and us not being able to do anything," said Goudsmit, describing Lange as "a compassionate man".
"He dedicated his whole life to HIV."
- 'Ahead of his time' -
"He has been the architect and principal investigator of several pivotal trials on anti-retroviral therapy and on the prevention of mother-to-child transmission of HIV," said the Amsterdam Institute for Global Health and Development, of which Lange was the executive scientific director.
His partner, Van Tongeren, was the institute's head of communications.
Lange had served on several public and private advisory boards, had published more than 350 scientific papers, and was chief of drug development at the World Health Organization's Global Programme on AIDS.
He had created pilot programmes to get poor people on AIDS treatment, getting private companies to fund his work.
"His ideas were always ahead of his time," said Cooper.
"Joep was the very first person to say that if you could get a cold Coke in any village anywhere in sub-Saharan Africa, there was no reason why you couldn't distribute anti-retroviral therapy to everyone who needed it."
Michel Kazatchkine, the UN's special envoy for AIDS in Eastern Europe and Central Asia, said he was devastated by the news.
"Joep Lange was one of the most dedicated, honest, gentle, knowledgeable colleagues in the AIDS field that I ever met," he said on his Twitter account.
French AIDS researcher Francoise Barre-Sinoussi, awarded a Nobel medicine prize for co-discovering HIV, said the loss of Lange and colleagues was a "tragedy" for the scientific community.
According to Cooper, Lange was an avid reader and lover of art, who "always had a book in his hands".
He leaves behind four daughters and a son.