This short note describes the relevant details of formulating and implementing general bilinear quantity-quality balances found in industrial processes when data reconciliation is applied. The modeling also allows for the straightforward generation of analytical first-order derivatives. Quantity-quality balance problems are those that involve both extensive and intensive stream variables such as flows and compositions, respectively and are related through laws of conservation of material, energy and momentum. The balance equations involve both linear and bilinear terms (multi-linear) of quantity and quality where quantity times quantity and quality times quality are not germane although they can be included easily. Two numerical examples are provided to demonstrate the new formulation technique. (C) 2003 Elsevier Ltd. All rights reserved.