Computer modeling techniques have been used to study the structure and energetics of H defects in Mg2SiO4 forsterite and their infrared stretching frequencies. The calculated defect binding energies indicate that OH groups will combine with cation vacancies to form neutral defect complexes. Such defect clusters are stabilized by strong hydrogen bonding. Calculated infrared stretching frequencies for OH in different environments have been determined for the first time and show two distinct groups of frequencies. The values of these frequencies, together with their directional components, are compared to experimental measurements. This comparison shows that in pure forsterite, H defects will be associated with both Si and Mg vacancies, but will not occur as isolated OH groups, in line with the results from consideration of defect energetics.