Enzymes, also known as biocatalysts, have numerous advantages over chemical catalysts and because of that their use and demand in pharmaceutical industry is constantly growing. The enzymes that catalyze reactions of nitrile hydrolisis into carboxylic acids and ammonia without the formation of amide intermediates are called nitrilases. Nitrilases are used in conversion reactions (R)-4-chlorine-3-hydroxybutyronitrile into (R)-4-chlorine-3-hydroxybutyric acid. Afretwords, a spontaneous reaction of cyclisation (R)-4-chlorine-3-hydroxybyric acid into (R)-3-hydroxy-γ-butyrolactone occurs, a versatile chiral building block, often used in synthesis of numerous pharmaceutical polymers and solutions. In this paper, the stability of nitrilase during the synthesis of (R)-4-chlorine-3-hydroxybutyric acid was monitored. The synthesis of (R)-4-chlorine-3-hydroxybutyric acid takes place in a batch reactor. The activity of nitrilase at different contrentrations of the substrate, (R)-4-chlorine-3-hydroxybutyronitrile, over a period of 24 hours was measured with the spectrophotometric assay. The experimentally obtained data was compared with those predicted by the mathematical model. It was observed that the activity of nitrilase decreases with time and it was concluded that deactivation of nitrilase occurs. The deactivation constant at different substrate and enzyme concentration was estimated from the obtained experimental data and it was concluded that the initial conditions in the reactor affect the stability of nitrilase.