A Similitude Law for Leaching Systems has been developed based on a one-dimensional mass transport theory which includes the processes of diffusion, sorption, and radioactive decay. The similitude law can be used to determine the leaching characteristics of a solidified waste from a laboratory leaching test conducted for a similar test piece and to determine the long-term leaching characteristics of a test piece from the results of a short-term test. A leaching number developed from the similitude law was found to be the key parameter in characterizing leach test systems. A mathematical model based on one-dimensional transport theory was also developed to determine the general leaching characteristics of a solidified low-level radioactive waste in a test leachant. The results were normalized, based on the simililtude law, in a leaching graph which uses the leaching number as a system parameter. The leaching number can be obtained through short-term leaching tests. The normalized leaching graph can be used to determine the leaching characteristics of a solidified waste without undergoing computer simulation or tedious numerical summations of infinite series.