A thin substrate approximation is used to derive closed form solutions for the Green's functions Ge and Gm appearing in the mixed potential integral equation modeling of planar microstrip structures. The expression for Gm is given in terms of the contributions from a source and its image, while that of Ge is derived in terms of the so-called quasi-dynamic contribution plus a ''correction'' term. Excellent agreements with the exact numerical integration of the corresponding Sommerfeld integrals for a wide range of parameters are obtained. In particular, it is demonstrated that the addition of the correction term is needed in order to accurately predict the sharp dip and its associated phase reversal that appear in the magnitude of Ge in the near field. ¿ American Geophysical Union 1995