Software architecture evaluation methods aim at identifying potential maintainability problems for a given architecture. Several of these methods exist, which typically prescribe the structure of the evaluation process. Often left implicit, however, are the concrete system attributes that need to be studied in order to assess the maintainability of implemented architectures. To determine this set of attributes, we have performed an empirical study on over 40 commercial architectural evaluations conducted during the past two years as part of a systematic ldquoSoftware Risk Assessmentrdquo. We present this study and we explain how the identified attributes can be projected on various architectural system properties, which provides an overview of criteria for the evaluation of the maintainability of implemented software architectures.