The top of a reservoir is often a seismic interface of decreasing velocity. No critical angle exists in reflections from such an interface, and so Zoeppritz reflection coefficients are closed-form and accurate at all incident angles and frequencies. However, most existing AVO methods use approximations to the Zoeppritz equations. These approximations assume small contrasts and small angles, and the number of invertible parameters is usually limited to two or three (the so-called two- or three-term AVO). We propose using the Zoeppritz equations for amplitude inversion of target reflections without critical angles. The Fréchet derivatives are calculated analytically. We use a linearized iterative least-squares inversion scheme. This algorithm is applicable to PP, PS, SS, and SP reflections. We demonstrate that PP amplitude data can be invertéd for four parameters (three velocity ratios and the density ratio), although joint inversion of PP and PS reflections can greatly improve the robustness. The algorithm is superior to conventional approximations in that it works for any large (decreasing) contrasts at any large angles; it is accurate and can invert for more parameters.