This paper investigates the diffusion and institutionalization of change in formal organization structure, using data on the adoption of civil service reform by cities. It is shown that when civil service procedures are required by the state, they diffuse rapidly and directly from the state to each city. When the procedures are not so legitimated, they diffuse gradually and the underlying sources of adoption change overtime. In the latter case, early adoption of civil service by cities is related to internal organizational requirements, with city characteristics predicting adoption, while late adoption is related to institutional definitions of legitimate structural form, so that city characteristics no longer predict the adoption decision. Overall, the findings provide strong support for the argument that the adoption of a policy or program by an organization is importantly determined by the extent to which the measure is institutionalized — whether by law or by gradual legitimation.