Cayman's Malachite Green Phosphate Assay provides a fast, reproducible, colorimetric method for measuring inorganic free phosphate in aqueous solutions. The assay method is based on the formation of a complex between malachite green molybdate and free orthophosphate that absorbs at 620-640 nm. Applications for this assay include quantification of phosphorylation and phosphate release from protein phosphatase substrates. This assay is a reliable and suitable means of detecting and quantifying minimal amounts of inorganic free phosphate and is amenable to high-throughput screening applications. The assay is formatted to a 96-well format, but could easily be modified for use in 384-well or cuvette-based assays.