A three-parameter adsorption equation theta =N/B= c1x(1+ c2x)+ c2x/1 + c1x was derived on the basis where the monolayer is a localized adsorption and the molecules in an adphase over the monolayer behave like a compressed gas in a different potential from that of the gas phase. This equation tends to the Huttig-type adsorption equation when c2 = 1.O. This equation shows a good fit for the experimental isotherms classified as extended type 1 over nearly the whole range of relative pressures.