Observations collected in April 1998 using the Canadian Convair-580 during the First International Satellite Cloud Climatology Project (ISCCP) Regional Experiment-Arctic Cloud Experiment (FIRE.ACE) are used to study cloud microphysics over the Arctic Ocean. Cloud microphysical parameters in climate models are specified as either constants or specific relationships based on cloud systems originating from either the ocean or land. The Arctic Ocean during winter and spring is mainly covered with ice. Because of this condition, the influence of the Arctic Ocean on cloud systems can be very different as compared to that of the midlatitude ocean. Air mass back-trajectories calculated from the Canadian Meteorological Center (CMC) model outputs were used to define the origin of air masses as either the Pacific Ocean (PO) or the Arctic Ocean (AO). The uncertainty in specifying the origin of the air mass was less than 20%. The PO mean aerosol number concentration (Na) from the aircraft measurements was larger (108 cm-3) than for the AO cases (41 cm-3). The PO mean droplet number concentration (Nd) was 48 cm-3 in comparison to 77 cm-3 for the AO cases. The droplet effective radii (reff) for the AO and PO cases were 9.3 and 5.6 ¿m, respectively. Liquid water content (ice water content) changed from 0.05 (0.01) g m-3 for the AO cases to 0.13 (0.03) g m-3 for the PO cases. The averaged ice crystal number concentration was 20 L-1 for the PO cases and 10 L-1 for the AO cases. For April 1998, a statistical significance test on mean values at the 85% confidence level showed that the AO and PO cases had distinct microphysical and aerosol characteristics.