<![CDATA[This research used Research Development (R D) by Borg and Gall that was aimed to: describe validity, describe the responses of teacher, and determine the validity of items, reliability, and level of difficulty of science process skill (KPS) assessment intrument on colloidal system. This assessment instrument was in the form of essay that measures basic KPS of students. The results of expert validation of content suitability, construct, and legibility aspects were 87.00%, 76.00%, and 85.56% respectivelyin high to very high categories. Teachers responses of the content suitability, construct, and legibility aspects were 92.00%, 88.00%, 90.00% invery high categories. Empirical tests showed that the assessment instrument had moderate item validity, very high reliability, and moderate level of difficulty.Penelitian ini menggunakan metode Research Development (RD) menurut Borg dan Gall yang bertujuan untuk : mendeskiripsikan validitas, mendeskripsikan tanggapan guru, menentukan validitas butir soal, reliabilitas, dan tingkat kesukaran asesmen KPS pada materi sistem koloid. Instrumen asesmen ini berkatagori tes tertulis dalam bentuk essay yang mengukur KPS dasar siswa. Hasil validasi ahli pada aspek kesesuaian isi, aspek konstruk, dan aspek keterbacaan adalah 87,00%, 76,00%, dan 85,56% berkatagori tinggi sampai sangat tinggi dikatan valid. Tanggapan guru pada aspek kesesuaian isi, aspek konstruk, dan aspek keterbacaan adalah 92,00%, 88,00%, 90,00% berkatagori sangat tinggi. Uji empiris menunjukan asesmen memiliki validitas butir soal sedang, reliabilitas sangat tinggi, dan tingkat kesukaran sedang.Kata kunci: instrumen asesmen, keterampilan proses sains, sistem koloid]]>
