In this undergraduate physical chemistry laboratory experiment, students acquire a high-resolution infrared absorption spectrum of carbon dioxide and use their data to show that the rotational-vibrational state populations follow a Boltzmann distribution. Data are acquired with a mid-infrared laser source and infrared detector. Appropriate absorption peak assignments are made by comparison of the experimental spectrum with the high-resolution transmission molecular absorption (HITRAN) database. Integrated absorption peak areas are used as a measure of relative rotational-vibrational state populations. The effect of temperature on the population distribution is explored by sampling the absorption spectrum at several different temperatures, giving students the opportunity to observe the properties of a Boltzmann distribution. (Contains 3 figures and 1 table.)