A newer fast, convenient and inexpensive quantitative method for the determination of thiourea using photochemical exchange reaction of sodium nitroprusside has been investigated. Sodium nitroprusside is a photolabile complex and it undergoes photochemical ligand exchange reactions rapidly. Some efforts have been made to utilise such reactions for the estimation of some sulphur containing anions and electron rich organic molecules. The progress of the reaction has been observed spectrophotometrically. The effects of different parameters like pH, change of concentration of sodium nitroprusside, concentration of ligands, light intensity etc. on percentage error was investigated. The efforts were made to minimise the percentage error and some optimum conditions were obtained. Such reaction can be used for the determination of thiourea in the range of millimoles to micromoles and hence, it is important to know whether such estimations can be done successfully and that to with the desired accuracy.