We consider the computation of an upper bound to error probability in Trellis-Coded Modulation (TCM) schemes in sequence dependent channnels. In its most common form, the pairwise-state" technique is applicable to this class of channels. However, it entails a degree of complexity that may not acceptable, because the dimension of the matrices involved in pairwise-state calculations is the square of the number of states, and the number of states, say N, equals the product of the code states and of the channel states. In this paper, we derive an error probability bound based on a computational technique which avoids the consideration of N2N2 matrices. It is based on the computaion of the transfer function of a graph with N1 nodes, and whose branch labels are NN matrices rather than scalars. we also describe some looser bounds which simplify the computation of error probability over intersymbol interference (ISI) channels, and show numerical results of the methods applied to trellis coded 16-QAM with 2-symbol ISI.