Two-phase flow regimes in tube bundles have been determined in vertical upward cross-flow of air and water in horizontal tube bundles. Experiments were conducted using staggered and in-line tube bundles with a pitch-to-diameter ratio of 1.47, containing respectively 26 and 24 rows of five 12.7 mm O.D, tubes in each row. A resistivity void probe was used to measure the local void fraction and the probability density function (PDF) of local void fraction fluctuations was used in an objective statistical method to determine the two-phase flow regimes. For the in-line tube bundle, a bubbly flow regime occurred for J(G) < 0.4-0.8 m/s and an intermittent Row regime for J(G) between 0.4-1.0 m/s and 3.9 m/s at all liquid flow rates tested (J(L) < 1.0 m/s). An annular flow regime occurred for J(G) > 3.9 mis at only low liquid flow rates (JL < 0.25 m/s). Compared to the previous flow regime maps, the intermittent flow regime was found to persist up to much higher liquid flow rates, and this was attributed to the difference in the flow regime identification methods used. The limitation of the visual observation technique for flow regime identification in tube bundles became evident as significantly different amplitudes of void fluctuations were measured and different flow regimes were shown to exist near the shell wall and inside the tube bundle. For the staggered tube bundle, the results were similar except for bubbly-intermittent flow regime transition which occurred at higher gas flow rates compared to the inline tube bundle, possibly due to more efficient break-up of large gas slugs.