The subnanosecond relaxation dynamics of two dithiacarbocyanine dyes, 3,3’-didodecyldithia-2,2’-carbocyanine bromide (dye I) and 9-methyl-3,3’-dihexadecyldithia-2,2’-carbocyanine perchlorate (dye II), were examined in dichloromethane solutions by picosecond transient absorption spectroscopy. Contrary to earlier reports, intersystem crossing to an isomerized excited state, together with internal conversion, plays a major role in the radiationless deactivation of excited dyes. Torsional motion of the polymethine chain is prerequisite to both processes. Intersystem crossing occurs at a rate comparable to that of internal conversion but is faster than fluorescence by an order of magnitude : k(isc) = 2.3 X 10(9) s(-1) (I) and 1.8 x 10(10) s(-1) (II); Phi(isc) = 0.55 +/- 0.05 (dye I) and 0.63 +/- 0.04 (dye H); tau(S1) similar to 241 +/- 55 ps (dye I) and less than or equal to 30 ps for the meso-substituted dye II. NO ground-state geometrical isomers formed in our time window (less than or equal to 12 ns); we have inferred that isomerization takes place from the longer lived triplet state manifold of these dyes.