Illite age analysis (Pevear, 1992) and similar mixing methods demonstrate near-linear behaviour when corrected for smectite content in illite-smectite and substitutions for K+ in illite layers. Srodon (1999) found that highly curved mixing lines result from theoretical calculations and challenged the validity of IAA. However, our theoretical calculations indicate near-linear mixing lines result when corrected for the amount of smectite and known K+ substitutions (Na+ or NH4+). We conclude that perfect linearity is unattainable because IAA attempts to unmix a system linearly, but the system is based on an exponential function (K-Ar equation). In practice, we believe IAA can successfully determine the age range of illite diagenesis, even though perfect linearity is not attainable because: (1) significant deviations from linearity occur only if >500 Ma separate the diagenetic and detrital ages; (2) IAA analytical uncertainties result in diagenetic and detrital ages that span a range of ages.