Recent studies suggest that teachers' emotions can influence their teaching. Guided by a phenomenological orientation, we used a qualitative single-case study approach to investigate the interplay between a teacher's emotions and the enactment of her instructional goals. We looked at Lily's (pseudonym) assessments of the success and failure of employing epistemological messages for sixth-grade students' sense-making and construction of scientific explanations using microsociological and exploratory interpretations. The study is important because it shed light on the crucial roles that teachers' emotions play in the success of their decisions to use reform-based teaching practices. Formal and informal interviews, classroom video recordings, field notes, and observation data from the six-lesson unit on "Plant structures and functions" were all included in the analysis. We used the control-value theory of achievement emotion to repeatedly code the data in order to investigate how Lily appraised the success and failure of employing epistemological messages. Additionally, we used the four dimensions of sound scientific explanations--relevance, conceptual framework, causality, and appropriate level of representation--to deductively evaluate the productivity of her epistemological messages. Results showed two opposing emotions, "joy" and "irritation", resulting in productive and unproductive epistemological messages. When she felt "joy" as a consequence of a favourable assessment of her instructional goals and expectations based on student outcomes, her epistemological messages effectively conveyed the majority of the dimensions of good scientific explanations. These findings reveal the importance of teachers' emotions during the implementation of their intended reform-based teaching practices and offer implications for future studies.