Part II of this series showed that at constant temperature and mass fraction a linear relationship exists between the number of carbon atoms and bubble/dew-point pressure of linear alkanes in supercritical propane. This paper investigates the extent to which this linear relationship can be extrapolated. In the crystalline phase linear alkanes undergo molecular folding, yet information on molecular folding in the fluid state is inconclusive. Molecular folding influences the nature of the alkane and it may thus influence the supercritical phase behaviour and thus result in a change in the linear relationship between the number of carbon atoms and the bubble/dew-point pressure of alkanes in supercritical propane. A model is proposed for molecular folding in supercritical solution. Measurements were conducted with a wax mixture containing an average of 102 carbon atoms and results showed that in the low wax mass fraction and mixture critical region the linear relationships hold true and molecular folding most probably does not occur. In the high wax mass fraction region lower pressures than predicted are observed and possible explanations of the lower than expected pressures include the polydisperse nature of the mixture and the possibility that partial molecular folding may have occurred. (C) 2007 Elsevier B.V. All rights reserved.