The May 26, 1998, measurements by millimeter-wave imaging radiometer (MIR) onboard the NASA ER-2 aircraft over the arctic region north of Alaska are analyzed to study the characteristics of cirrus clouds. The brightness from the 1.88 and 10.4 μm channels of the MODIS airborne simulator (MAS) and the 1.064 μm backscatter from the cloud lidar system (CLS) are used to identify these clouds. The brightness temperature depressions derived from the 340 and 183.3 ± 7 GHz measurements at 43° incidence are compared with the results of radiative transfer calculations to arrive at an estimation of ice water path (IWP) and median equivalent mass sphere diameter (D _me). The measurements at a high incidence angle of 43° are used to minimize the effect of surface emission. Two different particle size distributions are assumed in these calculations. The estimated average D _me values are ∼270 and ∼241 μm, and the average IWPs are ∼174 and ∼86 g/m² for the two distributions. The large difference in the estimated IWP values occurs in the domain of radiative transfer calculations at D _me ≤ 100 μm. There are also a number of pairs of 340 and 183.3 ± 7 GHz brightness temperature depressions that occur in the same domain, and the estimation of IWP and D _me cannot be made. These difficulties are probably caused by the low sensitivity to cirrus cloud detection at 183.3 ± 7 GHz.