An in situ nondestructive probe for measuring the concentration and distribution of helium and hydrogen isotopes in near-surface layers of solids is described. The technique makes use of the 3He(d, p)4He resonant nuclear reaction. A probing beam of deuterons is used to analyze for 3He (a 3He beam may be used to analyze for deuterium). Stopping-power effects control deceleration of the probing beam and bring the energy into the resonance window. Integration of the proton yield at a given beam energy determines the quantity of 3He or deuterium at the depth associated with the probe upon entering resonance. Spatial resolution of the method is defined by the 350-keV full width at half-maximum for the resonance cross section. Examples of ion-implanted 3He distributions in niobium are presented along with a discussion of mathematical-deconvolution and oblique-incidence resolution-enhancement techniques. Normal-incidence resolution is ~ 3 μm and can be improved using mathematical deconvolution. Oblique incidence (75° from the normal to the surface) in conjunction with deconvolution, yields adjacent-peak-position resolution to nominal values of 3000 ÅA by artificially broadening the distribution of 3He without changing the resolution function. Small-angle multiple scattering and energy straggling of the probing beam under these conditions introduces a limit on peak-width resolution of about 6000 ÅA. Differential cross sections for the reaction range between 50 and 80 mb/sr (depending on observation angle), which results in minimum observable concentrations of about 10 ppm. The method has technical value for analyzing potential problems associated with the implanation of helium in materials that may be used for the primary containment wall of controlled-fusion reactors.