The development of an instrument capable of in-situ measurements of thin film thickness and growth rate for multiwafer planetary MOVPE reactors has been investigated. The design of this instrument has necessitated detailed analysis of optical access to the wafers, with particular attention focused on aspects relating to the limited optical access to the wafer surface inherent in these reactor designs. These limitations determined how the instrument was designed in order to attain the best optical access to the wafer surface. Further experiments undertaken, during reactor operation, provided solutions on how best to collect reflectance data from the wafer surface, and also allow for consideration of the different processes occurring within the reactor, which would impact on the ability to make in-situ measurements. This led to the study of parasitic deposition using surface science techniques to determine the nature of the deposit, which obscures optical access to the wafer surface. These experiments complimented in-situ measurements that were made on film thickness and growth rate with good agreement between pre-determined layer thickness and values measured by ex-situ techniques. Finally experiments were undertaken using photoluminescence, which determined that the optical access used in one of the reactor designs has a detrimental affect on the more complicated optoelectronic device structures such as vertical cavity surface emitting lasers (VCSELs).