Scanning spreading resistance microscopy has been used to delineate the implant isolated regions of an InP optoelectronic device. This scanning probe microscopy-based technique provides direct insight into the effect of helium implant isolation on the electrical conductivity of the material. The nanometer-scale resolution of the technique allows us to directly image the implanted region of the device structure. The results of the scanning probe measurements are compared with theoretical simulations of the helium implant damage. The optimal implant isolation conditions were determined by direct measurements of the series resistance across implanted and reference device material. The scanning spreading resistance microscopy results were able to provide fundamental insight explaining the implant conditions required for optimal isolation. (C) 2004 American Vacuum Society.