Quantitative high-resolution transmission electron microscopy was used to study the distribution of indium atoms in In x Ga1− x N alloys by strain mapping. In GaN/In x Ga1− x N/GaN quantum wells with x < 0.1 we find that the sample thickness and the precision to which displacement fields can be extracted from a lattice image determine whether or not it is possible to discriminate between random alloy fluctuations and cluster formation. In miscible alloys such as SiGe or AlGaN a precision of better than 1 pm is required to reveal random alloy fluctuations, which presently exceeds experimental capabilities. In In x Ga1− x N with x > 0.1, a precision of about 3 pm suffices to distinguish random alloy fluctuations from indium clusters that are present. Thick In x Ga1− x N layers with x = 0.6 and x = 0.7 show phase separation with a wavelength between 2 and 4 nm and a fluctuation amplitude of Δx = 0.10 and 0.15, respectively. This produces striped composition fluctuations, which are modulated by dot-like structures. The similarity of the fluctuation magnitudes in quantum wells and thick layers suggests that spinodal decomposition occurs in both materials and our results place the centre of the miscibility gap around x = 0.5–0.6.