Electronic Journal of Differential Equations, Vol. 2006(2006), No. 04, pp. 1-10. Title: Regularization and error estimates for nonhomogeneous backward heat problems Authors: Duc Trong Dang (Hochiminh City National Univ., Vietnam) Huy Tuan Nguyen (Hochiminh City National Univ., Vietnam) Abstract: In this article, we study the inverse time problem for the non-homogeneous heat equation which is a severely ill-posed problem. We regularize this problem using the quasi-reversibility method and then obtain error estimates on the approximate solutions. Solutions are calculated by the contraction principle and shown in numerical experiments. We obtain also rates of convergence to the exact solution. Submitted November 11, 2005. Published January 11, 2006. Math Subject Classifications: 35K05, 35K99, 47J06, 47H10. Key Words: Backward heat problem; ill-posed problem; contraction principle; quasi-reversibility methods.