Nitric oxide (NO) can either boost or impede the growth of cancer cells depending on its concentration. Therefore, any anticancer treatment using NO requires precisely controlled NO administration to the target cells in terms of dosage and timing. In this context, photochemically activated NO donors were actively explored, but their detailed NO-releasing dynamics, which is crucial for their use, is not known yet. We determined detailed photoexcitation dynamics of a stable, nontoxic, and water-soluble NO precursor, cysteine-bound Roussin's Red Ester (Cys-RRE), including secondary reactions of the nascent photoproducts. The primary quantum yields of the NO dissociation from the photoexcited Cys-RRE were found to be 24-54% depending on the excitation wavelength; however, the geminate rebinding of NO with the nascent radical reduced the level of biologically available NO to as low as 12%. Such information is useful to achieve efficient NO delivery to practical chemical and biological targets.