Electron spin resonance (ESR) spectra of surface and interface recombination centers recently observed on (001) silicon wafers, labeled P m and KU1, were studied using spin dependent microwave photoconductivity. Both ESR spectra, having the orthorhombic symmetry and spins S ¼ 1/2 and S ¼ 1 for P m and KU1, respectively, were observed in the commercially available surface oxidized (001)-Si wafers. Systematic studies on annealing and oxidation conditions for the P m and KU1 formation conclude that both ESR spectra arise from the same center that contains the interaction between the two nearest Si dangling bonds on the (001) Si surface.