The study of an isomeric A/B mixture of the title oxime 1, by photolytic or thermal E,Z‐isomerization and NMR measurement including 1H{1H}‐NOE difference spectra, led to assignment of the E configuration to its predominating form A. The 1H/13C data were interpreted in terms of steric overcrowding of both forms, especially of the thermolabile photoproduct B. Four classical (empirical) NMR methods of elucidating the oxime geometry were critically tested on these results. Unexpected vapor‐phase photoconversion A→B in the window glass‐filtered solar UV and spectroscopic findings on their protonated states were discussed, as well. The kinetically controlled formation of the N‐protonated species (Z)‐5+ was proved experimentally. In addition, some 1H NMR assignments reported for structurally similar systems were rationalized (3 and 4) or revised (1 and 7–9) with the GIAO‐DFT(B3LYP) and/or GIAO‐HF calculational results. Copyright © 2007 John Wiley & Sons, Ltd.