In this work a causal P-matrix method for leaky SAW(LSAW) devices and the determination of the thereto needed COM-parameters slowness s and reflection r are presented. As a consequence of causality s and r in general are complex. The imaginary parts of reflection and slowness are interpreted as the exchange of energy between SAW and BAW. A relation of energy conservation between imaginary parts of reflection and slowness is derived. The COM parameters are required to fit both the dispersion relation as well as the Harmonic Admittance. At the same time causality is required. In this way COM parameters are determined which are very similar to those from the Abbott-Hashimoto approximation [1]. In particular for metalization heights above 4%, however, the new model provides an improved loss description.