A general solution is given to the problem: With little computation, construct a more disaggregated input-output model from an available model, using information about some activities that were aggregated when the available model was built. The solution is exact and explicit. When only partial information is available, the exact solution guides its user to inexpensive and precise sensitivity analysis. Detailed consideration is accorded to the solution of the most frequently occurring problem--the disaggregation of one sector into two.