The scaling of bubble/slug formation in organic solvents at microscale without surfactant was initially investigated by using T-junction and symmetrically cross-shaped microfluidic devices. Four unique organic solvents and three dispersion methods were used, forming different flow patterns and dispersion size. The flow pattern of uniform slug flow was investigated. Both the gas-liquid flow and dispersion size, which ranged from 400 to 1400 mu m in length and 270 to 430 mu m in diameter, depended on several factors including dispersion method, two-phase flow rates, physical properties of the liquid phase, and structure of microchannels. A general equation L/w = k(Q(G)/Q(L))(alpha)Ca(beta) was used to characterize the dispersion size with modification of Q(G)/Q(L) for different dispersion methods, considering the influences of breakup rate and transformation of the interface shape on the dispersion process. Three models were developed to predict the dispersion size for different dispersion methods, and calculated data were in good agreement with the experimental results. (C) 2010 American Institute of Chemical Engineers AIChE J, 57: 2647-2656, 2011