This study investigated construct validity and factorial invariance of a performance assessment of reading comprehension and writing proficiency through a multitrait-multimethod (MTMM) structure using confirmatory factor analysis. The performance assessment was administered to 1,023 fifth and sixth graders. Interrater reliability was examined for each measured variable using three different generalizability coefficients. Although all of the measures were found to be highly reliable, exploratory factor analysis indicated that trait and method effects were confounded in the measured variables. Consequently, confirmatory factor analysis was used to disentangle multidimensionality and examine the convergent and discriminant validity of the latent variables according to the Campbell-Fiske criteria. A model with three correlated trait factors and three correlated method factors (MTMM structure) provided the best fit to the data. Factorial invariance across gender and grade was supported only for a particular set of parameters. Methodological and practical implications of the use of confirmatory factor analysis in MTMM analyses are also discussed for construct validation in performance assessment across different groups. (Contains 1 figure, 11 tables, and 36 references.) (Author/SLD)