ABSTRACT Chemical warfare agent (CWA) analysis faces a significant challenge when degradation or precursor compounds lack reference spectra in databases. This study focuses on three structural isomers, N ‐methyl‐ N ‐propylethanimidamide, N ‐methyl‐ N ‐isopropylethanimidamide, and N,N ‐diethylethanimidamide, which are relevant hydrolysis by‐products/precursors of Novichok nerve agents (Schedule 1.A.13/1.A.14) yet are absent from standard spectral libraries. All three isomers were synthesized and then comprehensively characterized by gas chromatography–electron ionization mass spectrometry (GC‐EI‐MS), gas chromatography–chemical ionization mass spectrometry (GC‐CI‐MS), and liquid chromatography–high‐resolution mass spectrometry (LC‐HRMS). Distinctive fragmentation signatures were identified for each isomer. Trimethylsilyl derivatization of the ethanimidamides enhanced GC performance and revealed additional diagnostic ions. LC‐HRMS results confirmed the fragment formulas and allowed unambiguous differentiation based on key ion intensity ratios. This work provides the first systematic mass spectral data for these Novichok‐related ethanimidamides. The value of these reference spectra was demonstrated in the 57 th OPCW Proficiency Test (PT‐57): an unknown spiking chemical was successfully identified as N ‐methyl‐ N ‐propylethanimidamide by comparison to the generated spectra. The study thus delivers new analytical capabilities for CWA forensics—enabling confident identification of challenging isomeric degradation products through a combination of tailored derivatization, multimode mass spectrometry, and expert interpretation of fragmentation pathways.