The conformational equilibria of neutral serine are studied by experimental matrix-isolation Fourier transform infrared spectroscopy in combination with density functional theory (DFT) calculations. The geometries and energies of the low-energy conformers of serine were optimized using the DFT(B3LYP)/6-31++G** method. In addition, we calculated the infrared frequencies and intensities of the most stable conformers in order to assist in the assignment of the vibrational bands in the experimental spectrum. The calculated relative energies suggest that four conformers are sufficiently stable to appear in the gas phase and all could be distinguished in the experimental matrix infrared spectra. We also calculated theoretical rotamerization constants and compared these with experimental determined constants. For the equilibria SER2/SER1 and SER3/SER1, a deviation between the experimental rotamerization constant and the theoretical constant was found. A relatively strong intramolecular H-bond in conformers SER2 and SER3 is at the origin of this discrepancy.