A novel, selective acylation of primary phenylalkyl amines (PPAAs) using hexamethyldisilazane (HMDS) and perfluorocarboxylic acids (PFCAs) is noted. Couples, like HMDS and trifluoroacetic acid, HMDS and pentafluoropropionic acid, or HMDS and heptafluorobutyric acid trigger PPAAs’ quantitative acylation. Processes’ selectivity was characterized by applying all couples to derivatize benzyl, 2-phenylethyl, 3-phenylpropyl, 4-phenylbutyl amines, and their relevant substituted versions. Aliphatic amines were unreactive. Identification, quantification, proportionality, and stoichiometry in derivatization processes were determined by gas chromatography/mass spectrometry. Reaction conditions were optimized depending on reagents’ molar ratios, solvents, and temperatures applied. The new acylation method, in comparison to the traditional ones, obtained with trifluoroacetic anhydride, heptafluorobutyric anhydride, and N-methyl-bis(trifluoroacetamide), offers numerous advantages. Derivatives, provided by couples, can be directly injected onto the column, avoiding loss of species, saving time, work, and cost in the preparation process. Due to traditional reagents’ excess evaporation by nitrogen drying, the loss of trifluoroacylated species proved to be 65% or less. Regarding heptafluorobutyryl species, their losses varied between 25% and 5%. Unified huge responses, obtained with the HMDS and PFCA couples are attributable to their direct injection onto the column and to fragments sourced from the molecular ions and from their self-chemical ionization ([M]•+, [M+147]+, i.e., [M+(CH3)2–Si═O–Si–(CH3)3]+). The reaction mechanism, due to the HMDS symmetrical structure, acting HMDS as acylation generator for PFCAs, was confirmed by density functional theory (DFT) computation.