The anaerobic digestion (AD) of fat-containing waste is often prolonged in time and problematic. Differences in AD performances could rely in a different probability for microorganisms to access the substrate. The aim of this study was to study the AD of waste cooking oil (WCO) with a biophysical approach. Two laboratory experiments were carried out using model systems consisting of WCO + hydration medium (HM) in 100 mL, static, in-batch reactors. In the first experiment, we assumed the WCO to HM (OtoW) ratio as an indicator of the accessibility of substrate to microorganisms: the higher the ratio, the greater the probability of feeding for the microbial cells. AD performances were evaluated in relation to 5 decreasing OtoW ratios. In the second experiment, we favored the formation of emulsions through alkalinisation, by adding to our model system 5 increasing amounts of KOH 1M (pH range from 6.7 to 10.1). High OtoW ratios (that is, relatively low volumes of aqueous phase) increased the CH4 production rate while allowing CH4 yields close to the theoretical. However, the highest OtoW ratio resulted in AD failure. A proper amount of alkali halved the time to join the maximum CH4 production. Reasoning in terms of biophysical factors, more than in terms of oil concentration or inoculum-to-substrate ratio, could be helpful for the improvement of AD of fat-containing substrates.