Hydroxyl terminated oligo(ε-caprolactone) (OCL) monolayers were reversibly cross-linked forming two dimensional networks (2D) at the air-water interface. The equilibrium reaction with glyoxal as the cross-linker is pH-sensitive. Pronounced contraction in the area of the prepared 2D OCL films in dependence of surface pressure and time revealed the process of the reaction. Cross-linking inhibited crystallization and retarded enzymatic degradation of the OCL film. Altering the subphase pH led to a cleavage of the covalent acetal cross-links. The reversibility of the covalent acetal cross-links was proved by observing an identical isotherm as non-cross-linked sample. Besides as model systems, these customizable reversible OCL 2D networks are intended for use as pH responsive drug delivery systems or functionalized cell culture substrates.