Vibrational transition moment directions for 7-methyladenine (7MA), 9-methyladenine (9MA), and 2-amino-9-methylpurine (2A9MP) have been determined from infrared linear dichroism measurements on samples partially oriented in stretched poly(vinyl alcohol) (PVA) films. The sign ambiguity problem for the experimental directions of 9MA and 2A9MP was in part solved by exploiting the change in orientation character of the ribose derivatives adenosine and 2-aminopurine riboside. In the case of all transitions in 7MA and most transitions in 9MA and 2A9MP, polarizations calculated in self-consistent-reaction-field (SCRF) DFT(B3LYP)/6-31G(d,p) calculations were used to guide the choice of experimental polarizations. The SCRF spectra were generally in better agreement with experiment than the corresponding calculated gas-phase spectra. The IR spectrum of adenine in PVA film was analyzed in terms of contributions from the 7-H and 9-H tautomers, and the relative abundance of the minor 7-H tautomer in PVA film was determined to be 23 +/- 3% at room temperature.