Isotope enrichment is widely used to affect atomic masses, facilitating data acquisition and peak assignments in experiments such as nuclear magnetic resonance and infrared spectroscopy. It is also used for elucidating the origin of weak features in systems where natural isotopic abundances are low. However, it is not possible to always know a priori precisely how vibrational modes change for arbitrary levels of isotopic substitution. Here, we examine this issue by presenting a joint experimental and theoretical study for the important case of ¹³C isotope substitution effects on the infrared spectra of calcite. By systematically varying the ¹³C : ¹²C ratio, we find that the relative positions and intensities of infrared-active vibrational modes can vary, in a non-linear and mode-dependent fashion, with minority isotope content and proximity. This allows us to determine the origin of weak spectral features due to the natural abundance of isotopes and to show that even relatively low levels of substitution are not necessarily within the “dilute limit,” below which isotopic substitutions do not interact.