The order parameters (S) of a number of substituted azobenzenes and phenylazothiazole, -benzothiazole and -heterocyclic compounds dissolved in a nematic liquid crystalline host have been determined. The measured order parameters for a class of azo compounds bear a strong relationship to the structure of the dye. For azobenzenes, there is an additivity of substituent effects which leads to good agreement between empirically-derived and observedSvalues. Disazo dyes give higherSvalues than the corresponding monoazo dyes. For heterocyclic phenylazo dyes, the end group substituents on the phenyl ring play a significant role in influencingS, whereas the nature of the heterocyclic system, as well as the end group substituents on the heterocyclic ring, are only of minor importance. The presence of an N-H moiety as part of a heterocyclic ring, or the fusion of a benzene ring onto a heterocyclic ring, can lead to significant changes in the order parameter.