Rocking disposable bioreactors are a newer approach to smaller-scale cell growth that use a cyclic rocking motion to induce mixing and oxygen transfer from the headspace gas into the liquid. Compared with traditional stirred-tank and pneumatic bioreactors, rocking bioreactors operate in a very different physical mode and in this study the oxygen transfer pathways are reassessed to develop a fundamental mass transfer (k(L)a) model that is compared with experimental data. The model combines two mechanisms, namely surface aeration and oxygenation via a breaking wave with air entrainment, borrowing concepts from ocean wave models. Experimental data for kLa across the range of possible operating conditions (rocking speed, angle, and liquid volume) confirms the validity of the modeling approach, with most predictions falling within +/- 20% of the experimental values. At low speeds (upto 20rpm) the surface aeration mechanism is shown to be dominant with a kLa of around 3.5hr(-1), while at high speeds (40rpm) and angles the breaking wave mechanism contributes upto 91% of the overall kLa (65hr(-1)). This model provides an improved fundamental basis for understanding gas-liquid mass transfer for the operation, scale-up, and potential design improvements for rocking bioreactors.