AbstractIsomeric ring substituted monofluoro‐(E)‐3‐phenylpropenals (cinnamaldehydes) were synthesized in order to provide, with the NMR‐active19F nucleus, an insight into the influence of substituents and the transmission of electronic effects between the aromatic ring and the side‐chain. Their1H,13C and17O NMR chemical shifts and theirnJ(H,H),nJ(H,F),nJ(C,H) andnJ(C,F) values were determined. Generally, thenJ(C,F) coupling constants correlate with the length of the coupling route and thus simplify the assignments of the13C NMR spectra. Only the side‐chain carbons C‐2 and C‐3 in theortho‐substituted compounds showed an exception to this rule. Many long‐rangenJ(H,F) (n= 4–7) values were easily observed. The17O NMR chemical shift of the carbonyl oxygen in theo‐ andm‐fluoro derivatives show clear substituent effects, whereasp‐fluorocinnamaldehyde shows a value comparable to t