Air CO2 partial pressure (Pco2) in one of the monitored parts in Lascaux cave (Hall of the Bulls, near the entrance) is variable. For the year 2009, three major components of Pco2 were identified: a seasonal component, a low-frequency component and a high-frequency component. As a first step to understand the presence of Pco2 in caves, this study aims to separate these components and quantify their contributions to the Pco2 signal. Seasonal component is calculated with a sinusoid curve fitted to the measured Pco2. This component is subtracted from the measured Pco2. Then, from the remaining value of Pco2, low- and high-frequency components were separated using Fourier transform, threshold filtering of the frequency and Inverse Fourier transform. Seasonal component is the main contributor (82 % on the average); high-frequency component participates 12 % on the average. Low-frequency component is not present all the year; the average participation is 6 %. Finally, to link these three components with other measured parameters such as temperature, water flow and atmospheric pressure, hypotheses were proposed.