Despite expectations from the literature, clean Pt{311} is shown to exhibit a (1 x 2) reconstruction, and CO adsorption leads to a lifting of this reconstruction to a (1 x 1) Pt structure. The reconstruction and its lifting are similar to those observed for CO on Pt{110}. A (1 x 1) island nucleation process occurs above a critical coverage of around 0.16 ML. Sticking probabilities and calorimetric adsorption heats are reported, and from the data the energy difference between Pt{311}-(1 x 1) and (1 x 2) is estimated to be in the order of similar to 10 kJ mol(-1). (C) 1999 Elsevier Science B.V. All rights reserved.