New monomer fraction data (fraction of non-hydrogen bonded molecules) of dilute (x(solute) < 0.016) C-1 to C-3 alcohol-in-acetone and dilute acetone-in-alcohol systems were collected via Fourier transform infrared (FTIR) spectroscopy. Monomer fraction data may be used to improve regression parameters within the Statistical Associating Fluid Theory (SAFT) modeling framework. For very dilute (x(alcohol) < 0.003) alcohol-in-acetone mixtures, it was found that 2-propanol had the highest monomer fractions and methanol the lowest. As the alcohol mole fraction increased (x(alcohol) > 0.003), methanol maintained the lowest monomer fraction (X-mon,X-MeOH = 0.01 at x(MeoH) = 0.0086), with ethanol, 1-propanol and 2-propanol approaching similar monomer fraction values, i.e., X-mon,X-alcohol -> 0.06. For dilute acetone in alcohol, and especially for methanol and ethanol, there was a pronounced trend towards acetone monomer fractions of 1 at infinite dilution. The acetone monomer fractions decreased according to an exponential decay function to values of +/- 0.3 for acetone dissolved in methanol and +/- 0.1 for the other alcohols investigated. Acetone monomer fractions, therefore, tended to decrease as alcohol chain-length increased, showing that acetone could more easily penetrate the hydrogen bond network of the solvent when the solvent/solvent hydrogen bonds were weaker. For dilute acetone in 2-propanol, a previously unrecorded monomer peak was observed and quantified. (C) 2015 Elsevier B.V. All rights reserved.