Contraction, rather than expansion, during heating occurs more frequently in monoclinic minerals than in those of other systems. The deformation during heating was measured for LiVO₃ (diopside-type structure) and ZrO₂ (baddelyite), and discussed for wolframite-hubnerite. In six out of seven monoclinic structural types the deformation consists of a shift of the angle of monoclinicity toward 40o, i.e. towards higher symmetry. The cause of the negative thermal expansion is shear deformation. -R.A.H.