The purpose of the study is to explore Turkish preservice science teachers' informal reasoning regarding socioscientific issues and the factors influencing their informal reasoning. The researchers engaged 39 preservice science teachers in informal reasoning interview and moral decision-making interview protocols. Of the seven socioscientific issues, three issues were related to gene therapy, another three were related to human cloning, and one was related to global warming. The data were analyzed using an interpretive qualitative research approach. The characteristic of informal reasoning was determined as multidimensional, and the patterns of informal reasoning emerged as rationalistic, emotive, and intuitive reasoning. The factors influencing informal reasoning were: personal experiences, social considerations, moral-ethical considerations, and technological concerns.