A process-engineering model is presented for the stratified-wavy-to-intermittent (SW-I) flow-pattern transition in slightly inclined gas - liquid pipe flow The main parameter for predicting (in)stability of wavy flow in inclined pipes is the average liquid holdup, which was found to reach a maximum critical value at flow-pattern transition. Observed values of the critical liquid holdup vary between 0.07 and 0.42, depending on pipe diameter, angle of inclination and transport properties of the gas-liquid system. Measurements were performed in transparent glass pipes of 26- and 51-mm dia., at ten angles of inclination (0.1 degrees less than or equal to beta less than or equal to 6.0 degrees), using air/water and air/tetradecane (n-(C1)4H(30)) systems at atmospheric pressure. Flow-pattern maps ave presented for selected angles of inclination, showing excellent agreement between predicted and observed flow-pattern boundaries.