A simple X-ray back-reflexion method is proposed for studying small variations in lattice parameters. This method minimizes the random errors in a reproducible and objective manner. It also eliminates the film-to-sample distance measurement, as used by some workers [Megaw, (1933). Proc. Roy. Soc. A 142, 198-214] in determining the lattice parameters. This is of great advantage where one wishes to study changes in lattice parameters with temperature. Its limitation is that it gives the lattice parameters only in relation to the room-temperature values and not their absolute values.