We investigated and compared the learning environment perceptions of students, teachers and guides who participated in Science, Technology, Engineering and Mathematics (STEM)-based outreach activities in secondary education. In outreach activities, schools and teachers work together with companies and other external institutions in learning activities in order to motivate students for the STEM domain. In this study, we identified characteristics of outreach activities that explain variance in perceptions of students. Data were gathered from 729 high-school students as well as 35 teachers and guides in 12 activities both in the US and the Netherlands. A questionnaire was used to asses outreach activities based on subscales from validated questionnaires such as the What Is Happening In this Classroom, Constructivist Learning Environments Survey, Classroom Environment Scale and the Learning Climate Questionnaire. Teachers' perceptions were more positive than students' perceptions for most scales, while guides perceived the outreach learning environment in almost the same way as students. Student perceptions were very positive for outreach activities. Outreach activity characteristics such as teaching method and emphasis were found to be the most important factors in explaining variance in students' perceptions between activities. Long-term problem-based activities and the perspective of new views of science and scientists were perceived as providing the most positive learning environments. Additionally, outreach learning environments can create opportunities to increase students' motivation in STEM.