<![CDATA[This study examines the effects of Coretax System Implementation (X1), Technology Adoption based on the Technology Acceptance Model (X2), and Adaptation Burden (X3) on Taxpayer Compliance and Efficiency (Y) in Surabaya following full implementation of the Core Tax Administration System (CTAS). A sequential explanatory mixed-methods approach was employed. Quantitative data were collected through structured questionnaires from 120 modern taxpayers, comprising corporate and professional individual taxpayers, while qualitative validation involved in-depth interviews with 10 key informants, including corporate CFOs, certified tax consultants, and tax officials. Multiple Linear Regression analysis using IBM SPSS included data quality tests, classical assumption tests, t-tests, F-test, and coefficient of determination analysis. The results demonstrate that X1, X2, and X3 simultaneously have a significant effect on taxpayer compliance and efficiency (F = 184.25; p < 0.001), explaining 82.2% of the variance (Adjusted R² = 0.822). Partially, Coretax System Implementation positively and significantly affects compliance and efficiency (β = 0.154; t = 4.052; p < 0.001), reflecting benefits from integrated processes such as Taxpayer Account Management and prepopulated data. Technology Adoption is the strongest predictor (β = 0.782; t = 17.378; p < 0.001), confirming that perceived usefulness and ease of use support voluntary compliance. Adaptation Burden has a positive but statistically insignificant effect (β = 0.042; t = 1.024; p = 0.308). Overall, the findings indicate that efficiency gains from Coretax substantially offset transitional technical frictions and internal ERP adjustment costs. These findings highlight the importance of user-centered implementation strategies, training, and continuous digital support for taxpayers.]]>
