Despite the importance of DNA/RNA hybrids, a mesoscopic approach is still missing. We developed a mesoscopic model for DNA/RNA hybrids and obtained the complete parametrization from published melting temperatures. The new parameters show very clearly the reduced strength of the hydrogen bond of dArU base pairs, resulting in a weakened stabilization of the double helix and a strong stability dependence on deoxypyrimidine content. The stacking parameters show no correlation to B-DNA, however we obtain a moderate correlation to A-RNA. We discuss some example sequences with variable deoxypyrimidine contents and for polipurine tracts (PPT).