The radical intermediate, which is produced in the process of electroless nickel plating, was detected in the presence of water-soluble trap α-4-pyridy1-N-tert-buty1 nitrone（POBN） or α-phenyl-tert-buty1 nitrone（PBN） by electron spin resonance（ESR） spectroscopy. The results obtained indicated that a 16-line（g=2.006 1, aβC—H=1.98×10-4 T, aγP—H=3.32×10-4 T, aN=aP=15.93×10-4 T） or a 18-line （g=2.004 9, aβC—H=aγP—H=2.38×10-4 T, aP=17.72×10-4 T, aN=16.20×10-4 T） ESR spectrum was observed in the process of electroless nickel plating when spin trap PBN or POBN was added. The spectra-line patterns and hyperfine coupling parameters were greatly indentical with those in the solution composed of Ti3+（0.01 mol/L）+H2O2（0.05 mol/L）+H2PO-2（0.01 mol/L）+POBN or PBN（0.01 mol/L）. Therefore, the spin adducts formed in the process of electroless nickel plating could be ascribed to the adducts belonging to HPO-·2 and POBN or PBN and it follows that the anion radical HPO-·2 was really produced from the oxidation of sodium hypophosphite in the process of electroless nickel plating.
