Hafnium and zirconium atoms react with 14,202 molecules and with H-2 + O-2 mixtures to form M(OH)(2) and M(OH)(4) molecules, which are trapped in solid argon and identified from isotopic shifts in the infrared spectra. Electronic structure calculations at the MP2 level converge to almost linear M(OH)(2) and tetrahedral M(OH)(4) molecules and predict vibrational frequencies for mixed isotopic molecules of lower symmetry that are in excellent agreement with experimental measurements, thus substantiating the identification of hafnium and zirconium dihydroxide and tetrahydroxide molecules. Titanium atoms react to give the same product molecules, but Ti(OH)(4) has an S-4 structure with bent Ti-O-H bonds, Ti(OH)(2) appears to be nearly linear, and the more stable tetravalent HM(O)OH isorner is more prominent for Ti. The Group 4 tetrahydroxides reported here are the first examples of pure metal tetrahydroxide molecules.