Singlet biradicals of the 3,4-dimethyleneheterocycle series, where the heterocycle is furan, thiophene, or N-substituted pyrrole, with chromophores in the UV-vis region (560-640 nm), are generated by nanosecond laser flash photolysis of diazene precursors in fluid solution. The rate constants for disappearance of these transients are determined by time-resolved absorption spectroscopy using the long-wavelength band. The rate constants for dimerization are near the encounter-controlled rate. Rates of trapping by alkenes are well described by a frontier orbital model. Oxygen trapping rate constants range between 10(7) and 10(9) M-1 s-1. The ratios of rate constants for trapping of 3,4-dimethylene-N-(p-toluenesulfonyl)pyrrole by pairs of alkenes in preparative competition experiments are in good agreement with those determined by the flash photolytic method. This confirms assignments of the 3,4-dimethyleneheterocycle structure to the chromophore of the biradical transients.