In this paper we develop a procedure to estimate the opacity of Martian dust storms from the spectral measurements made by the Viking infrared thermal mapping instrument. Methods are also produced to estimate the surface temperature and the effective temperature of the dust cloud from these observations, in conjunction with the lander opacity observations. The derived opacities, for various locations on the planet, show the basic trends identified in the lander measurements. At the height of the second dust storm we derive 9-μm opacities of ~3, which correspond to values of ~5 at visible wavelengths. The analysis of zonally averaged data for the southern hemisphere at the time of the second major dust storm indicates dust opacities of ~2--3 remaining at 1800--2400 LT, with the maximum opacity occurring in the late afternoon period.