Cathy Pharoah, the Charities Aid Foundation's director of research, has left to set up her own consultancy.
Pharoah has formed a new research company - Third Sector Prospect - that will work with donors, government and universities.
"I'm carrying on the same sort of work that I have been doing and hope to contribute to research and debate on philanthropy, giving and the sector's resources," she said.
She will also undertake projects with CAF.
Pharoah joined CAF in 1995 as head of research. She is the fifth director of CAF to have left in the past 12 months. Policy director Simon Hebditch, donor services director Jenny Byers, business services director Alan Jeffs and philanthropy services director Sharon Stearnes have also departed.