It has been shown that Fourier transform infrared (FTi.r.) microscopy is able to monitor the phase morphology in blends of poly(methyl methacrylate) and poly(styrene-co-acrylonitrile). This is carried out by the spatially resolved detection of the absorbances of the C=O stretching vibration of the ester group of poly(methyl methacrylate) at 1732 cm(-1) and the C=N stretching vibration of poly(styrene-co-acrylonitrile) at 2238 cm(-1). The most serious restriction for FTi.r. microscopy is the large i.r. spot size which limits the spatial resolution of the detectable morphologies. An enormous advantage of FTi.r. microscopy compared to optical microscopy is its ability to provide quantitative information on the chemical composition of the phases. These data taken at different annealing temperatures can be used to obtain the phase diagram of blends showing lower critical solution temperature behaviour. Larger deviations of data points measured by FTi.r. microscopy compared to cloud point measurements are observed at the poly(methyl methacrylate)-rich side of the phase diagram, where also the error of the FTi.r. microscopic determination of the phase composition is larger for the blend system under investigation.