The use of thin‐film interferometry to determine the thickness of thin films is extended from unpatterned to patterned films. We developed a two‐color thin‐film interferometer which is robust and suitable for shop‐floor use. With this new diagnostic, the etch rates of both the masking film and the underlying layer can be monitored. Several approaches to analyzing the thin‐film interferometry data from patterned samples are explored including a mechanistic model, a simplified sine wave model, and fast Fourier transforms. The relative advantages and disadvantages of each technique are described. Both the mechanistic and sine wave models are complicated by the fact that they are nonlinear functions of the etch rates. It was therefore useful to examine maps of the mean squared error of the fit of data to the sine wave model. The analysis techniques are applicable to samples that have one or more thin films, are patterned with features that are larger than the wavelength of light, λ, and where at least a thickness λ of each film is etched away. The analysis techniques are demonstrated on laboratory data sets of the etching of photoresist‐masked oxide films on silicon wafers.