The Microwave Sounding Units (MSUs) aboard TIROS-N/NOAA satellites have played an important role in providing long-term global climate information. However, mainly due to the limb-correction and the intrascan averaging procedures in the data processing, changes to the characteristics of the data may have occurred. In this paper we investigate this possibility in two parts. In the first part we have chosen to revisit the radiative transfer theory and to re-derive the (effective) radiative transfer equation for the processed MSU data under more general conditions. We find that the weighting function of the processed MSU deviates from the original at nadir and that the normalization condition, which ensures the total weight of contributions to the total observed radiance from different sources sums up to unity, is breached. In the later part of the paper we generate simulations of climatological MSU-2 before and after data processing using a line-by-line radiative transfer code of Shah and Rind <1995> with the monthly mean output of the National Centers for Environmental Prediction (NCEP)/National Center for Atmospheric Research (NCAR) reanalysis project <Kalnay et al., 1996> for the period 1982--1994. These simulations are examined and compared with the processed MSU-2 observations available in the public domain. We find that a systematic reduction of contrast between land and ocean MSU-2 brightness temperatures can result from the limb-correction procedure. The temporal variances are affected by the greater-than-unity limb-correction coefficients, especially in the tropical region. A reduction of the spatial variances over the land region, as was reported earlier by Basist et al. <1995>, can be caused by the intrascan averaging treatment in the processing. We suggest that a better approach in future MSU data processing may be to use the nadir data directly without averaging with the other off nadir data and, in addition, that the off-nadir data should be navigated and be processed into separate data sets. ¿ 1998 American Geophysical Union