Phosphatidylglycerol (PG) is an anionic phospholipid, a constituent of biomembranes and an important component of drug delivery formulations and modern vaccines. Its mixtures with phosphatidylcholine (PC) exhibit non-linear properties, which are usually attributed to the molecular immiscibility of PG and PC. Here, using a combination of spectroscopic (FTIR, fluorescence) and computational (quantum chemistry, molecular dynamics) approaches, we show that the nonlinear properties of PG/PC mixtures are caused by hydrogen bonding between PG molecules. The latter are able to change their conformation to either form or break intermolecular hydrogen bonds.