A range of analytic methods for quantifying free fatty acids (FFAs) in milk were compared and further correlated to rancid flavour detected by sensory analysis. Proton transfer reaction–mass spectrometry (PTR–MS) and 1H nuclear magnetic resonance (NMR) spectrometry were introduced as novel methods for quantifying individual short chain FFAs. Gas chromatography–mass spectrometry (GC–MS) combined with in-solution derivatisation was used for quantifying all individual FFAs, and Bureau of dairy industries (BDI), acyl-CoA-synthetase–acyl-CoA oxidase (ACS–ACOD) assay, Fourier transform infrared spectroscopy were used to estimate the total FFA concentration in 28 milk samples. High correlations (r between 0.76 and 0.86) to rancid off-flavour were found for all analytical methods, and all the individual FFAs were quantitatively inter-correlated. Furthermore, a high correlation was found between PTR–MS and GC–MS quantification of hexanoic acid. For the BDI method, the sensory thresholds rancid off-flavour in farm bulk milk were 1.95 mmol 100 g−1 fat.