A thermally stable double beam laser interferometer for the measurement of mechanical strains due to domain switching in ferroelectric materials is described. Sample sizes vary from 0.25 mm to 5 mm in thickness, and maximum strains produce from 1 to 50 fringes in the data. A quadrature detection scheme provides the resolution which is required to interpolate accurately the fringe record and determine the direction of surface mechanical displacement. Specific sample mounting techniques are also developed in order to attain well‐defined loading conditions. Two sample holders, one for mounting very thin discs and the other for 5 mm cubes, are described. They induce minimal mechanical loadings on the samples. Typical data exhibiting the effects of domain switching due to an applied electric field are shown for the hot‐pressed ceramic PZT 65/35.