The Massachusetts Water Resources Authority (MWRA), in coordination with the Southern Nevada Water Authority (SNWA), the Town of Concord, MA, and the Portland, Oregon Water Bureau, through an American Water Works Research Foundation (AwwaRF) Tailored Collaboration Project is evaluating the effectiveness of ozone followed by medium-pressure ultraviolet light (MP-UV), and of medium-pressure UV alone for disinfection of water and its effects on distributed water quality. This paper presents early results of the research which will determine: the effectiveness of ozone and UV for inactivation, in terms of log inactivation, of Giardia, Cryptosporidium, and human enteric viruses; the synergistic impacts of ozone and UV on disinfection byproduct formation, biostability, corrosion control, and red water; and, provide guidance for the design and operation of the optimum primary disinfection scheme. This study involves many water quality evaluations, but the crucial results are those that will affect the design and implementation of ozone and UV treatment processes in water treatment facilities. Preliminary results indicating the percent UV transmittance reduction following ozone and the potential for reduction in disinfection byproducts are presented, along with bench-scale test results of cold water impacts on Cryptosporidium and Giardia inactivation. Results of MS-2 challenge tests are also presented. Includes 6 references, tables, figure.