A composition of matter suitable for use in a reversible thermometer is disclosed which comprises a thermally responsive material capable of being supercooled at least for several minutes, and subject to a change in state from a solid to a liquid substantially at a predetermined temperature; means for visually observing the change in state; and a matrix forming material comprising an amorphous organic compound, the matrix forming material being insoluble in the thermally responsive material; whereby the thermally responsive material is dispersed within the matrix forcing material, the composition of matter being reversibly responsive to changes in temperature, and remaining in the liquid state for a time sufficient to permit a user of the composition to observe the change in state. The preferred matrix forming material is polyisobutylene, and the preferred thermally responsive material is a solid solution of ortho-chloronitrobenzene and ortho-bromonitrobenzene. A process is disclosed for preparing the composition of matter.