The side chain of phenolalkylamines containing a bifunctional group was derivatized as the cyclic boronate, and then the residual phenolic group was trimethylsilylated. The resulting derivatives were single reaction products in each case, with good gas chromatographic properties and informative mass spectra to afford prominent molecular ions by gas chromatography with electron impact mass spectrometry (GC/EI-MS). The cyclic boronated-trimethylsilylated derivatives yielded the isotope pattern for boron ( 10 B: 11 B = 1:4.2) and characteristic ions [M] , [M Γ H] , [M Γ CH 3 ] , [M Γ RBO] , [M Γ TMSO] , and [M Γ TMSO Γ C 6 H 5 ] ions in the mass spectra. In order to distinguish between m-and p-phenolalkylamines, the mass spectra of the cyclic phenylboronated-trimethylsilylated (PBA-TMS) derivatives were compared with those of the trimethylsilylated (TMS) derivatives. The TMS derivatives of octopamine (p-) and norfenefrine (m-) showed identical mass spectra, while the PBA-TMS derivatives had mass spectra sufficiently different from one other to distinguish between the isomers. The most prominent ion of the PBA-TMS derivative is the [M Γ H] ion (m/z 310) for octopamine and the [M] ion (m/z 311) for norfenefrine.