Hydrotalcites with different counterions were tested with respect to their ability to act as heat stabilizers in PVC. The stabilization activity results from the capacity of the layered double hydroxides (LDHs) to react with the HCl formed during degradation of PVC. The experiments described are heat stability tests and experiments for a determination of the capacity of the hydrotalcites to react with HCl gas. They show a linear relationship between HCl capacity of LDHs with different counterions and their action as (heat) stabilizer. The reaction between the LDHs and the HCl occurs in a two-step process, firstly (if possible), the counterions between the LDH layers tend to react with the HCl gas, and, secondly, the LDHs themselves react with the HCl under complete destruction of the LDH structure and formation of metal chlorides.