In rotating biological contactors (RBC), the oxygen mass transfer coefficient (K(L)a) is often inadequately predicted by the available models. Hence, dimensional analysis based empirical models were developed for predicting KLa and the component of KLa due to turbulence (K(L)a(t)) using data available in the literature. The overall oxygen transfer number (OTN) and its component due to turbulence (OTN,) were defined as dimensionless groups based on K(L)a and K(L)a(t), respectively. They were expressed as a function of dimensionless groups, formed using disc diameter (D), area of discs (A(d)), rotational speed (omega) and cross-sectional area of the tank (A(t)). OTN was also a function of thickness of the water film on the disc (delta) and working volume of the reactor (V). Decrease in number of discs and decrease in (A(d)/A(t)) resulted in decreasing OTN but increasing OTNt. Both OTNt and OTN increased with increase in omega. The proposed empirical models will facilitate scale-up of RBCs. (c) 2007 Elsevier Ltd. All rights reserved.