Electronic Journal of Differential Equations, Vol. 2020 (2020), No. 35, pp. 1-11. Title: Existence and uniqueness of the p-generalized modified error function Authors: Julieta Bollati (CONICET, Argentina) Jose A. Semitiel (Univ. Austral, Rosario, Argentina) Maria F. Natale (Univ. Austral, Rosario, Argentina) Domingo A. Tarzia (CONICET, Argentina) Abstract: In this article, we define a p-generalized modified error function as the solution to a non-linear ordinary differential equation of second order, with a Robin type boundary condition at x=0. We prove existence and uniqueness of a non-negative $C^{\infty}$ solution by using a fixed point argument. We show that the p-generalized modified error function converges to the p-modified error function defined as the solution to a similar problem with a Dirichlet boundary condition. In both problems, for p=1, the generalized modified error function and the modified error function are recovered. In addition, we analyze the existence and uniqueness of solution to a problem with a Neumann boundary condition. Submitted April 19, 2019. Published April 18, 2020. Math Subject Classifications: 34A34, 47H10, 33E30, 34A12, 35R35. Key Words: Modified error function; generalized modified error function; nonlinear ordinary differential equation; Banach fixed point theorem; Stefan problem.