Composites of organic molecules and molecular sieves are promising materials for nonlinear optics. In order to achieve different NLO processes in these materials we varied the guest molecules. Paranitroaniline (PNA) and dimethylaminobenzonitrile (DMABN) were investigated in different molecular sieve hosts (AlPO-5, ZSM-5, VPI-5). These two groups of materials studied can be described by qualitatively different models : ensembles of oriented two-level or three-level systems. In contrast to molecular sieves loaded with PNA which can only mix waves of the same polarization, for the DMABN based materials the mixing of waves of different polarizations is observed.