We wish to study the variation with temperature of the structure of the beta polymorph of metal-free phthalocyanine (H2Pc) at 15K, 100K and room temperature. If the expected change in occupation factor for the two "inner" H atoms, from almost ordered on two N's at 15K to disordered over all 4 inner N's at 295K, is confirmed, measurements above room temperature would also be undertaken. In addition to resolving the longstanding question of the nature of the NH tautomerisation, and providing a definitive answer as to whether cis or trans tautomers, or a mixture of the two, exist at each temperature studied, the accurate ADP's at 3 or more T's will be used for full Capelli-Burgi analysis of thermal motion. These results will also provide a reliable basis for future DFT studies on H2Pc and related macrocycles