A high selectivity catalyst start-up process is provided in which an excess level of chloride moderator (greater than 1 ppm) is present in the feed gas composition during each of the various stages of the start-up process. The excess level of chloride used in the start-up process maintains a low level of selectivity (less than 86%) during the entire start-up process. Despite the low selectivity values achieved during the start-up process of the present disclosure, high selectivity catalysts that are conditioned by such a start-up process exhibit improved catalyst performance during the normal operation of the catalyst.