Reactor physics properties of a series of Be0 233U assemblies were measured as part of a programme of development of a high temperature gas cooled reactor, five such assemblies with moderator to fuel ratios ranging between 737 and 6539 to unity were placed between the core tanks of the Argonaut type reactor MOATA. Each assembly occupied a volume of one cubic foot and was subcritical in isolation; inserted into MOATA the assemblies formed a two-region critical system. The report gives complete details of mechanical arrangements for the assembly components and for methods of emplacement in the reactor; it also discusses the limitations inherent in the availability of materials and the restricted volume for assembly location. Details are given of methods used for approach-to-critical and for calibration of control absorbers on each assembly Both theory and practical aspects of measurements made are covered in the text. Reference is made to differential neutron spectra by time-of-flight and chopper techniques; epithermal neutron spectra by activation of integral detectors, fission rate ratios by use of gas flow chambers, and reactivity measurements by long period sample oscillation. The methods employed are critically examined, and difficulties and disadvantages stated; in some cases alternative methods are compared. An account is given of theoretical calculations complementing the measured values. The limited volume available prevented asymptotic spectra being attained and the geometrical complexity led to serious difficulties in setting up the calculational model. The possibility of using discrepancies between experiment and calculation as a means of improving basic nuclear data and calculational methods was impaired by the compromises necessitated by the modelling difficulties. Nevertheless it is shown that experimental values could be reconciled with those calculated with the aid of some empirical adjustment, thus demonstrating internal consistency, the work served to highlight the known need for simple geometry for the study of such assemblies, and provided valuable experience in evaluation of experimental and data analysis techniques.