Physisorption: van der Waals forces, reversible, low enthalpy (20-40 kJ/mol), multilayer possible, non-specific (all gases on all surfaces), no activation energy, favoured at low temperature. Chemisorption: actual chemical bonds, usually irreversible, high enthalpy (80-240 kJ/mol), monolayer only, highly specific (particular gas on particular surface), may need activation energy, initially increases then decreases with temperature. At low temperature: physisorption dominates. As temperature rises: physisorbed molecules desorb, but chemisorption increases until optimal temperature, then decreases. Sometimes physisorption converts to chemisorption with increasing temperature.