Photophysical processes of merocyanine 540 (MC540) are studied using picosecond time-resolved emission spectroscopy. The nonradiative rate of MC540 increases from 0.32 x 10(9) s(-1) in dioxane to 8.73 x 10(9) s(-1) in water, i.e., nearly 27 times. This is attributed to the decrease in the barrier of: photoisomerization with increase in polarity and the consequent increase in the rate of pl lotoisomerization for MC540. The polarity-dependent isomerization is also shown to be the cause for the observed increase in emission intensity and lifetime of MC540 in aerosol-OT microemulsions and aqueous bovine serum albumin solutions.