The temperature effect on the surface pressure (π)-molecular area (A) isotherms and the texture of the condensed phase domains of 9-hydroxyhexadecanoic acid monolayers are studied. The features of the monolayer are drastically changed by alkyl chain substitution by an OH group in the 9 position. The 9-hydroxyhexadecanoic acid monolayers show unusual temperature behaviour of the π-A isotherms and striking shape changes of the condensed phase domains at different temperatures. The morphological features indicate molecular packing of non-tilted alkyl chains in an orthorhombic and hexagonal lattice, respectively.