A closed-form decomposition approximation for finding the data performance in voice/data queuing systems is presented. The approximation is based on Courtois' (1977) decomposition/aggregation techniques and is applied to Senet hybrid multiplexing, movable boundary frame allocation schemes. The approximation is applied to both infinite and finite buffer systems. In the former case the approximation is valid only in the underload region and serves as an upper bound for the mean data queuing delay. In the finite buffer case it is valid for the whole data traffic range and is shown to improve as the number of channels increase, and deteriorates as the buffer size increases. For finite buffer systems upper and lower bounds for the decomposition approximation have also been derived. It is found that the lower bound is tight in the underload and low traffic region of the overload. In these same regions the decomposition approximation serves as a tight upper bound. >