This research was aimed to find out:(1) the relationship between students' critical thinking skills with students' achievement;(2) the relationship between students' learning motivation with students' achievement; and (3) the relationship between students' critical thinking skills and learning motivation with learning achievement in learning about buffer solution. This was a quantitative research with a correlational approach. The populations were 178 students at eleventh grade science program from SMA Negeri 6 (senior high school) in Surakarta, Central Java, Indonesia, academic year of 2019/2020.. One class with 36 students was selected sample by using cluster random sampling. The data were obtained through a multiple-choice test about critical thinking skills, a multiple-choice test about the buffer solution topic, and a questionnaire about students' learning motivation. The data were analyzed by using Pearson Product Moment correlation, multiple correlations, and multiple regression analysis. The results showed that:(1) A positive and significant relationship between students' critical thinking skills with students' achievement was found, as shown by the correlation coefficient value of 0.871;(2) A positive and significant relationship between students' learning motivation with their achievement was found, as shown by the correlation coefficient value of 0.884; and (3) A positive relationship between students' critical thinking skills and learning motivation with their learning achievement, as shown by correlation coefficient value of 0.931. Regression analysis obtained the equation of Y= 10,416+ 0.423 X 1+ 0.487 X 2, and the value of coefficient determination was 0.845 or 84.5%.