Purpose: The purpose of this study was to examine the measurement structure of the linguistic features of speech-language pathologists' (SLPs) talk during business-as-usual therapy sessions in the public schools, and to test the longitudinal stability of a theorized dimensional structure consisting of quantity, grammatical complexity, and lexical complexity. Method: Seventy-five SLPs' talk during therapy sessions with primary-grade students was automatically transcribed and coded for linguistic features from a corpus of 579 video-taped therapy session videos collected at the beginning, middle, and end of one school year with an approximately 12-week interval. We explored video characteristics and conducted descriptive statistics on eight linguistic indices of SLP talk to examine the variability in SLP talk between therapy sessions. Confirmatory factor analyses were used to explore the dimensional structure of SLP talk at each timepoint separately for the theorized three dimensions, and we conducted longitudinal measurement invariance analyses to test the stability of the three-factor structural model across the academic year. Results: There were considerable variabilities among SLPs in the characteristics of SLP talk during therapy sessions. The proposed three-factor structure of SLP talk consisting of quantity, grammatical complexity, and lexical complexity had good model fit at all three timepoints. The linguistic measurement properties representing the three factors were invariant over time. Conclusions: Results provided robust evidence of between-SLP variability in their child-directed talk, established a three-dimensional structure of the linguistic features in SLP talk, and identified that the linguistic features in SLP talk stably measured the same constructs across one school year, based on measurement invariance. The dimensions of SLP talk during therapy with students may represent important, malleable features of therapy that influence child language gains. [This paper was published in "American Journal of Speech-Language Pathology" v33 n2 p866-882 2024.]