4-Nitrophenyl butyrate consists of butyric acid chains esterified with 4-nitrophenol groups, thus giving it a yellow color. This compound is commonly used as a substrate in enzyme assays to measure esterase and lipase activity. When these enzymes cleave the ester bond, the nitrophenol group is released and the color changes from yellow to orange. Thus, the rate of color change can be used to determine enzyme activity. In addition, 4-Nitrophenyl butyrate can also be used as organic synthesis reagent and dye intermediate.