Assessment in education involves collecting and processing information related to student achievement during learning. Assessment activities help educators to find out where learners are having difficulties which for the sake of completeness requires an integrated instrument. The assessment carried out should conform to 21st-century learning criteria pertaining to cognitive and affective aspects of learning, and involve the identification of difficulties related to higher-order thinking skills (HOTS) and scientific attitudes of students. The purpose of this research was to develop an integrated assessment instrument that could be used to measure students' HOTS and scientific attitudes in relation to a chemistry topic, reaction rates. The research uses a 4-D development model, which includes the stages of define, design, develop, and disseminate. The results of the research were analysed quantitatively and qualitatively using item response theory (IRT). Based on the content validity, the integrated instrument was declared to be valid according to Aiken's V equation. All items in the test instrument were valid based on the Rasch, item fit, and item difficulty. The integrated instrument has also been applied based on the reliability of the estimated test and test information function (TIF) curves used to determine the profile of students' HOTS and scientific attitude.