Analysis of high resolution neutron powder diffraction data of ferroelastic LaNbO4 indicates a monoclinic to tetragonal structural phase transition that is accompanied by anomalous line broadening. The origins of this line broadening have been investigated using a novel modification of the Rietveld technique. The results show that strain broadening is associated with the ferroelastic phase transition both above and below the transition temperature. This anomalous ‘microstrain’ is well-described by a second rank strain tensor, the principal axes of which are collinear with the spontaneous lattice strain. This phenomenon may be a general feature of proper ferroelastic phase transitions.