This study compares two motivation scales in relation to relevant external criteria such as the students' Math academic achievement and the motivation assessed by teachers. The first scale is based on the theory of the causal attribution and built concerning the more frequent singular causes of scholastic achievement such as effort, task, ability, interest, teacher and exams; the educational motivation scale is built on concepts, such as intrinsic and extrinsic motivation and amotivation. Their application to a sample of secondary students yields much more meaningful correlations for the attributional scale, and the multiple regression analysis shows a greater predictive power for the variables of attributional motivation than for those of the educational motivation scale. The students' motivation assessed by Math teachers depends intensively and directly on the students' achievement and still the attributional scale give more meaningful results as predictor of teachers' motivation. These results show the high predictive power about academic achievement of motivational variables that become an external validation criterion for both motivation scales.