A study of the reactivity of five thio-organic compounds with palladium II ions has been made and one of them, 1-thioglycerol, selected for use as a spectrophotometric reagent for palladium. Its sensitivity is 0.01 μg of palladium cm 2 for log I 0 I = 0.001. The sensitivity of the other four compounds is about the same. Beer's law is obeyed over the palladium concentration range of 0.5 to 9 ppm. The effects of pH, order of addition of reagents, temperature and diverse ions have been investigated.