Quasi-classical CASSCF trajectory calculations have been carried out on s-cis-1,3-butadiene and substituted 2,3-dideuterio-1,3-butadiene (DDB) to assess the inertial effect on the ultrafast nonadiabatic deactivation of their first singlet excited states. Calculations indicate that even this modest increase in the mass of the 2,3-substituents noticeably affects the photodynamics of cis -> trans isomerization, by reducing the efficiency of the vibrational energy leakage between the initial relaxation and subsequent nonadiabatic decay modes. In qualitative agreement with experimental findings on related 1,3-dienes, the slowing down of the intramolecular vibrational energy redistribution (IVR) upon substitution results in extended excited-state lifetimes and reorients the photoregioselectivity toward cis rotamers and cyclic products.