To determine why ninth grade students taking the "Minimum Basic Skills Test" (MBST) and the "California Achievement Test" (CAT) performed differently in relation to reading achievement, a study investigated whether the noted discrepencies in reading achievement on the tests were due to differences in vocabulary and comprehension (skills and readability) inherent in both tests. Subjects, 180 high school freshmen, were given the two tests in the spring of 1984. Readability on the comprehension passages of both tests was determined using the Fry Readability Formula. The analysis of the grade level of vocabulary items on both tests was made using the Harris-Jacobson Core List. Both tests were also examined to determine the skills being assessed as measures of comprehension. In addition, a correlation of the results of the tests was made to determine the percent of variance accounted for and, finally, significant differences between the adjusted means of the test measures were made. The results supported three conclusions: (1) there were some similarities between readability and skills assessed on both tests, but marked differences also existed; (2) the difficulty level of the words in the vocabulary subtests was considerably higher on the CAT than on the MBST; and (3) the two tests measured reading of different kinds. (Extensive tables of data are appended.) (HOD)