Triethylamine (TEA)water mixtures have a critical-temperature (Tc). Below Tc the mixture exists as one phase and above Tc it exists in two phases. The de-mixed volume is different to the mixed volume. A nanosecond pulsed-laser heated a TEAwater mixture so that it de-mixed. The resulting dynamics of volume expansion were monitored using interferometry. For T-jumps within the one phase region the dynamics of volume change were limited by the speed of sound. However, T-jumps between the one and two phase regions also manifested a slower volume change associated with the de-mixing process. After 150ns, the volume of the de-mixed TEAwater was consistent with the equilibrium volume change. This suggests that, within 150ns, the system had split into phase-domains having equilibrium compositions of TEA and water. Subsequently the phase domains would simply merge and grow resulting in no further volume change to reduce surface tension between the phases.