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From 1934 to 1938, Bethlehem Steel Corporation initiated a series of outdoor corrosion tests involving several hundred low-alloy steel compositions. These tests were conducted in the industrial atmospheres of Bethlehem, PA, Pittsburgh, PA, and Columbus, OH, for up to 16 years. The purpose was to define the effects of various alloying elements and impurities on the corrosion resistance of steel. Included were steels with variations in elements P, S, C, Mn, Si, Cu, Ni, Cr, As, Mo, Sn, V, W, Al, and Co. The results were used in the development of weathering steel alloys and to estimate the effects of subsequent changes in composition. Data from these tests were analyzed statistically, and a set of equations was derived that allow for the calculation of corrosion behavior as a function of the above elements. This was done by fitting the results for each alloy to an exponential equation (C = ATB), where C is corrosion loss, T is time of exposure, and A and B are constants. At each exposure site, constants A and B for all steels were then fitted to equations expressed as linear combinations of the alloying elements (i.e., A = a0 + Σaixi, where a0 is a constant, ai is a coefficient for element i, and xi is the concentration of element i). Examples are given demonstrating the use of these equations for calculating the corrosion loss as a function of time for steel with various combinations of the above elements, and for estimating how the corrosion losses of weathering steels are affected by changes in the content of a particular alloying element. Results show that P, Si, Cr, C, Cu, Ni, Sn, and Mo are beneficial to corrosion resistance. S is shown to have a very large adverse effect. V, Mn, Al, Co, As, and W have little, if any, effect on corrosion.