Abstract The accumulation of molecules in the P-isomer ground state of DODCI in ethylene glycol is studied experimentally and theoretically. The P-isomer S 0 state is populated by S 0-S 1 absorption of cw argon ion laser light or dye laser light at various wavelengths. The population is probed by the absorption of a weak He-Ne laser. The recovery to the thermal ground-state equilibrium of N and P isomers is determined by measuring the temporal probe light absorption after pump laser switch-off. A steady-state level population theory is derived and applied to determine P-isomer S 0-S 1 absorption cross sections and transfer efficiencies between N isomers and P isomers.