Due to tube enhancements being used to achieve higher process efficiencies, heat exchangers are starting to operate in the transition region of flow. The paucity of data, however, has the implication that no correlation exists for enhanced tube transition flow. This article, being the first of a two-part paper, presents adiabatic friction factor data for four enhanced tubes for fully developed and developing flow in the transition region. Three inlets were used for developing flows, namely square-edged, re-entrant and bellmouth inlets. It was found that, as in the case of smooth tubes, transition was affected by the type of inlet used, with transition being delayed the most for the smoothest inlet. Correlations were developed to predict the fully developed critical Reynolds numbers and friction factors in the transition region. The correlations predicted the critical Reynolds numbers on average to within 1% with a root mean square deviation of less than 8%, while transition friction factors were predicted with a mean absolute error of 6.6%, predicting 89% of the data to within a 15% error. (C) 2010 Elsevier Ltd. All rights reserved.