Alkylation of isobutane with 1butene was carried out in the liquid phase at 353 K on beta zeolite catalyst samples with varying framework Si/Al ratios and particle sizes. The specific external surface area determined with nitrogen physisorption and t-plot analysis is the key parameter to rationalize alkylation activity and stability. The catalytic activity of zeolite beta samples with specific external surface areas smaller than ca 280 m(2) g(-1) suffers from intracrystalline diffusional limitations. The critical particle size to avoid diffusional limitations is estimated at 14 nm. In the absence of diffusional limitations, the alkylate yield obtained during the catalyst lifetime is proportional to the number of Bronsted acid sites in the zeolite and amounts to ca 3 kg mole(-1), irrespective of the acid site density. The deactivation of the samples is explained by a site coverage model.