Biotites from metapelitic xenoliths included within trachytes from the Euganean Hills (Italy) were analyzed by single-crystal X-ray diffraction (XRD), electron microprobe (EMP), scanning electron microscope (SEM), secondary ion mass spectrometry (SIMS), and Mossbauer spectroscopy. These biotites are Ti-rich and occur in gneissic xenoliths that underwent regional high-T/low-P metamorphism, at about 750 degrees C, followed by pyrometamorphism during incorporation in the melt at temperatures close to 950 degrees C. Biotites are zoned, with TiO2 content ranging from 6.79 (cores) to 8.14 wt% (rims). SIMS measurements show that the H2O content is in the range 2.88-4.08 wt%. The simultaneous occurrence of high-Ti and high-H2O contents, and the main cation substitutions based on EMP analyses suggest that the role of Ti-oxy in these biotites is less important than Ti-vacancy and Ti-Tschermak substitutions. Single-crystal XRD confirms that the Ti-oxy exchange was indeed effective but not the dominant substitution mechanism. Based on our data and those taken from literature on petrologically well-constrained systems, we propose that there is a petrologic control on the type of Ti-substitution mechanisms. We consider two types of petrologic groupings for biotites: (1) group A consisting of biotites from H2O-free or H2O-poor petrologic environments (e.g., volcanic rocks, ultrabasic xenoliths, and crustal xenoliths in which biotite underwent incongruent melting): Ti substitution in these biotites occurs via Ti-oxy predominantly, or more specifically Fe3+-Ti-oxy; and (2) group B consisting of biotites from H2O-rich petrologic environments (e.g., metamorphic rocks and crustal granitoids): Ti-vacancy, or more specifically Fe3+-Ti-vacancy, is the dominant mechanism in them. It is concluded that during high-grade metamorphism the dominating type of Ti substitution in biotite is controlled by H2O activity.