The hetrochain model was applied to develop a matrix formula describing the weight-average molecular weights for a reaction system that involves simultaneous long-chain branching and crosslinking. The heterochain model describes the molecular architecture formed through the primary chain connection that follows the Markovian statistics; therefore, this matrix formula is valid regardless of the chemical and reactor systems used, as long as the Markovian nature is preserved for the chain-connection statistics. Application to free-radical (co)polymerization systems is described in subsequent research. The gel point is simply described as a point at which the largest eigenvalue of the matrix M, which defines the chain-connection statistics, reaches unity. (C) 2004 Wiley Periodicals, Inc.