Ambient aerosol number concentrations and size distributions were measured in both indoor and outdoor environments using two identical co-located and concurrently operated optical particle counters (OPCs). Indoor measurements were performed in a research laboratory, whereas two different locations were used for outdoor measurements; the sampling duration exceeded 12 hours and one hour respectively. Results from the two OPCs have been presented for eight size classes between 0.5 and 20 µm, represented by central value diameters 0.875, 1.5, 2.75, 4.25, 6.25, 8.75, 12.5 and 15 µm. Overall, for the six indoor and outdoor experiments conducted at different times of day, the mean particle count ratios from the two OPCs for the individual samples showed ±20% variation for indoor experiments and ±50% variations for outdoor experiments. Significant random departures of the mean ratios from unity at all size classes were noticed even for indoor sample periods exceeding 20 hours. However, the coefficient of determination (R2) for the plots of readings from the two OPCs indicated higher consistency for “fine” particles (0.5–3.5 µm) than for “coarse” particles (10–20 µm), with average R2 > 0.8 and R2 <0.5 respectively. Poisson counting statistics help to explain the divergence in the latter case where number concentrations were very low for the outdoor experiments. However, it cannot explain the divergence for indoor measurements where the concentrations were much higher. Increasing the averaging period reduced the scatter, especially in size classes with low number concentration. However, this procedure may lead to over-smoothing of data for environments with rapidly changing number concentration. These results indicate that, when two such analysers are used for comparative studies, the divergence between their responses may generate significant values of source contribution or deposition flux, even for nominally similar aerosol populations.