The levels of stress and strain experienced by the windings of a superconducting magnet during its manufacture and operation are dependent on the mechanical properties of the multifilamentary composite wire that the windings are comprised of. It is also influenced by the change in dimensions of the wire during the reaction heat-treatment stage of the manufacturing process. Using specimens cut from a spool of 1.5mm diameter niobium tin type superconducting wire, the influence of the heat-treatment process on the mechanical properties and dimensions have been investigated. The heat-treatment was carried out in an inert atmosphere using apparatus specially developed for the purpose. For heat-treatment durations up to that required to complete the reaction of the niobium filaments into Nb3Sn, the volume and diameter of the wires increased with increasing heat-treatment duration. The maximum increase was 2% and 0.8% respectively. The length of the wires decreased slightly for the shorter heat-treatments, but increased up to 0.33% for the longer ones. The mechanical properties were significantly different for specimens that had no heat-treatment compared to those that had only a short heat-treatment, one that was insufficient to convert much of the niobium into Nb3Sn. Before heat-treatment the bronze within the wires is in a work-hardened state, but gets annealed during the heat-treatment and this is probably the major cause of the change in mechanical properties. Apart from becoming more brittle, the mechanical properties do not change much for different durations of heat-treatment. This is quite remarkable considering that the composition changes dramatically with the length of the heat-treatment.