The aim of this study was to develop a Scientific Process Skills Test (SPST) in the context of ''Matter and its Nature''. It was investigated whether there was a predictive effect of demographical features and participating out-of-school learning opportunities across the 7th and 8th grade students' Scientific Process Skill (SPS) levels. Quantitative research methods were used in this study. Data were collected from 289 middle school students for the validity and reliability of the test. The SPST consisted of 30 items and entailed three sub-dimensions (''basic scientific process skills, causal scientific process skills and experimental scientific process skills). The reliability coefficient of the test was calculated using the KR-20 formula and was found to be 0.84. The SPST was applied to 472 middle school students with the aim of determining whether there is a predictive effect of gender, grade level, school location, parent's education level and participation in out-of-school learning opportunities across the 7th and 8th grade students' SPS levels. Multiple regression analysis was conducted to determine the effects of demographical features and out-of-school learning opportunities on students' SPS levels. As a result, it was determined that the grade level, gender or mother's education level were important predictor variables that affect middle school students' SPS levels. It was also determined that out-of-school learning opportunities such as participating in science fairs, designing projects or reading scientific journals had an important predictive effect on students' SPS levels.