A series of luminescent transition metal complexes using the pH-sensitive ligand 5-carboxy-1,10-phenanthroline has been synthesized and characterized. The complexes, based on Ru(II) and Re(I), show monotonic changes in both luminescent intensity and lifetime with pH values over the range 2 < pH < 9. The impact of various structural features on both the range of pH sensitivity and dynamic response was studied using both intensity and lifetime measurements. It was possible to predictably tune the pH sensitivity range over about 1.5 pK(a) units. While significant variation in the dynamic response range was observed, the correlation with structural features needs further study.