ABSTRACTTuning the degree of bond-length alternation in organic nonlinear optical materials is a powerful paradigm for the design of organic materials with large molecular hyperpolarizabilities (β). Several research groups have employed this paradigm in the design and synthesis of NLO materials incorporating donor-acceptor polyenes. Increased bond-length alternation in polyenes leads to decreased barriers to rotation about C=C bonds and, hence, increased conformational flexibility. Since the degree of bond-length alternation is solvent dependent, so is the degree of conformational flexibility. In an effort to probe the influence of conformational flexibility on NLO response, we synthesized a series of simple donor-acceptor polyenes that are either conformationally flexible (la, 2a, 3a) or rigid (1b, 2b, 3b). For each pair of molecules ZINDO sum-over-states calculations predict a larger value of βμ, for the conformationally flexible isomer, but EFISH measurements (CHCl3) display mixed results. Various explanations for this behavior will be considered.