Filling wood cells with polymeric materials offers great opportunities to improve wood strength. Some applications, such as flooring, may require densification on one side of the material only. Yellow birch (Betula alleghaniensis Britt.), sugar maple (Acer saccharum Marsh.) and red oak (Quercus rubra L.) were surface densified through lateral chemical impregnation of monomers under vacuum followed by in situ electron beam polymerization. Lateral impregnation led to low mean chemical retention of 4% for sugar maple, 11% for red oak and 12% for yellow birch. X-ray densitometry and microtomography revealed an asymmetric density profile comparable to mechanically surface densified wood due to polymer-filled vessels. Scanning electron microscopy images showed presence of polymer-filled fibers beneath the surface. Brinell hardness of all species increased significantly compared to untreated wood. Low chemical retention significantly improved hardness due to localized polymer beneath the surface. Density profile characteristics were extracted from densitometry curves and correlated to hardness. Deep monomer penetration was more favorable to hardness than high surface density peak.