Selection of collision energies by a time-of-flight method has been combined with Penning ionization electron spectroscopy by utilizing a multichannel scaler and 2 MB random access memory. This technique provides a two-dimensional Penning ionization electron spectrum (2D-PIES) in which the Penning electron intensity is observed as a continuous function of both collision energies and electron energies, The observed 2D-PIES for Ar/He* (2(3)S and 2(1)S) was compared with calculated 2D-PIES obtained by classical trajectory calculations based on interaction potentials and transition probabilities.