The problem of mass balancing flowsheets with partial observation of stream components is considered. Minimization of relative error squares of stream components is used to determine the optimum set of balanced stream components and stream mass splits. Since the missing components do not contribute to the objective function, some of the optimization Lagrangian multipliers are equal or zeros. Furthermore, it is shown that some of the multipliers are dependent on the rest of the multipliers. The multipliers and a set of independent stream mass splits are used to search for the optimum solution. For a given partially observed data set, the maximum number of missing stream components that will still permit flowsheet mass balance is determined. Published by Elsevier Science B.V.