The infrared interference method of measuring the layer thickness of silicon layers deposited on heavily doped substrates of the same type is a well‐known technique. The correction factors and theoretical models in existence are discussed and their limits of application defined. It is shown that, for most cases, it is not necessary to correct for the profile of impurities in the epitaxial layer due to out‐diffusion and autodoping. A new method of calculating the thickness from the interference maxima and minima is presented which includes the phase shift and wave‐length dependence of the reflection at the layer‐substrate interface. It is shown that, with a spectrophotometer capable of scanning the wave‐length range 2.5–50μ, layer thicknesses to 0.8μ can be measured. Comparisons are made between this technique, a stain technique, and the spreading resistance technique for N/N+ and P/P+ silicon with layer thicknesses from 1.5 to 16μ. The precision of the infrared interference technique is quoted and correlation curves between the techniques are presented.