Ethers and esters of cyanohydrins that contain optically detectable moieties are highly effective for detecting, monitoring, and measuring the activity of enzymes that cause the cleavage of certain types of substrates. The cyanohydrins function as proaldehydes and proketones, spontaneously converting to aldehydes and ketones, respectively, which provide a large increase in optical detectability relative to both the starting esters and ethers and the cyanohydrins.