The monolayer behaviour of 4-(10,12-pentacosadiynoicamidomethyl)-pyridine at the air-water interface was studied by measuring the surface pressure-area isotherms. The amphiphile formed stable monolayers with a clear liquid-expanded (LE) to liquid-condensed phase transition at various temperatures. Upon addition of transition metal ions to the subphase complexation at the air-water interface occurred causing an increase of the pressure (IIc) at which the phase transition appeared. With complete complexation the phase transition disappeared and the monolayer had only the characteristics of a LE phase. The amount of complexation depended strongly on the transition metal used. Moreover, the metal ion concentration, complexation time, temperature, counter ion and the ionic strength of the subphase had a great effect on the complexation behaviour of this amphiphile.