[en] Measurements by photoemission spectroscopy have shown that the chemical environments of alkaline earth cations in high temperature superconductor compounds are different from the normal environments of these ions in the prototype perovskite structures. A recently discovered HTSC compound (Pb₂Sr₂Y_O . 5Ca_O . 5Cu₃O₈) exhibits the exceptional feature of having a typical HTSC signature for Sr, while Ca shows a normal signature. The crystal-chemical features of the relevant compounds have been correlated with these observations. These correlations may be indicative of inter-layer electronic interactions. 16 refs., 3 figs., 1 tab