To provide a solid support for automated synthesis of 3'-(aminoalkyl)-modified oligonucleoside methylphosphonates, controlled pore glass beads were functionalized with a protected N-(3-aminoprop-1-yl)-N-(2-hydroxyethyl)-2-aminoethyl ester of succinic acid. This "Aha-CPG" was used for automated synthesis of oligo-2'-deoxyribonucleoside methylphosphonates having either of two distinct 3' terminal modifications. If the first coupling to the beads was of a base-protected 5'-(dimethoxytrityl)-2'-deoxyribonucleoside 3'-(beta-cyanoethyl N,N-diisopropylphosphoramidite) synthon, then, upon completion of methylphosphonate oligomer synthesis and deprotection, the 3'-[N-(3-aminoprop-1-yl)-N-(2-hydroxyethyl)-2-aminoethyl] phosphate] derivative of an oligonucleoside methylphosphonate was produced and was shown to be a stable structure which affords a primary alkylamine group suitable as a site for further conjugations. If the first coupling was of a 5'-(dimethoxytrityl)-2'-deoxyribonucleoside 3'-(N,N-diisopropylmethylphosphonamidite) synthon, the initial product of synthesis and deprotection underwent a spontaneous, regiospecific ester cleavage in aqueous solution to produce an oligonucleoside methylphosphonate 3'-(methylphosphonate). An application of the Aha-CPG to the synthesis of rhodamine-conjugated oligonucleoside methylphosphonates is described in a companion paper [Thaden, J. and Miller, P. S. (1993) Bioconjugate Chem., preceding paper in this issue].