Introduction: The present work reports the characteristics of an instrument measuring the degree of motivation that people possess to think critically. The "Critical Thinking Motivation Scales" ("CTMS") is based on a theoretical option that affords precedence to the perspective of motivation for over the perspective of dispositions. Motivation is understood as the expectancy/"value". This sound theoretical frame offers further possibilities for researching factors that affect the activation of cognitive resources for the acquisition and deployment of critical thinking. Method: In this study 470 university students enrolled in Psychology degree programs were evaluated with CTMS. Factorial structure, reliability, and discriminatory capacity of this test were analysed. Also it was calculated the correlations between the CTMS and a critical thinking test: CT_MSLQ. Results: The results reveal that the "CTMS" has factorial structure compatible with Eccles and Wigfield motivation model, good realiability levels and discriminatory capacity and significative correlation with CT-MSLQ. Discussion and Conclusion: This study shows that the "CTMS" has psychometric characteristics that endorse it as a valid and reliable test. It is also a tool for the study of the motivational factors of critical thinking that may be useful in pedagogic instruction and intervention. (Contains 1 figure and 5 tables.)