Nonaqueous ink jet inks and their long-time stability at different temperatures were studied using single particle optical sensing (SPOS). Samples ground for the same period but having different amphiphilic polymer additives used for steric stabilization were prepared. SPOS characterizes the particle size distribution at the high end of the particle diameter (>0.5 μm.). In the aging at different temperatures, the high-end size distribution exhibited changes ascribed to sedimentation and aggregation. In the absence of aggregation, the decrease of the particle volume was identified as originating from sedimentation; the dependence on particle diameter and solvent viscosity agreed with theoretical prediction. The aggregation appeared as a hump in the tail of the main peak in the size distribution. The hump was apparent for inks aged at a higher temperature and having more loosely adsorbed polymer additives.