Here we report an experimental and computational study of toluene clusters that were formed in a He supersonic jet. A tunable vacuum ultraviolet radiation source was used to measure the ionisation potentials of dimers, trimers and tetramers from the onset of their photoionisation efficiency curves. DFT calculations were performed for different structures of stacked and non-stacked dimers and trimers as well as for a fully-stacked tetramer. Through comparison of the measured and calculated ionisation potentials, we show that under our experimental conditions, toluene nucleation starts from stacked dimers and proceeds through non-stacked trimers and tetramers. (C) 2010 Elsevier B.V. All rights reserved.