The MESG Phosphate Assay Kit has been developed for measuring the activity of any Pi-generating enzyme using MESG reagent. The measurement of Pi is based on absorbance change of MESG by phosphate. In the presence of inorganic phosphate, MESG is converted to 2-amino-6-mercapto-7-methlpurine by purine nucleoside phosphorylase with absorption wavelength shift to red. This feature has been used to develop our convenient MESG phosphate assay kit, an alternative to hazardous radioactive methods. The MESG substrate gives an absorbance increase at 360 nm on phosphorylysis at pH 6.5-8.5. The assay is shown to quantitate phosphate at the final concentration as low as 0.2 μM. The kit has been used for monitoring ATPase activities. It can also be used for monitoring phosphatase activities.