Cation exchange capacity (c.e.c.) determined by methylene blue can be regarded as a useful soil mechanical test in its own right. When a large number of liquid limit (LL) tests have to be done to determine variation in clays of the same general type, as on an engineering site, economy of operation could be achieved by carrying out many c.e.c. tests and few LL tests. It is only when comparing clays of widely different types that correlation is poor.