Science undergraduates' timely degree completion (TDC) has become increasingly important for students themselves, universities, and society, due to issues such as cost, efficiency, and productivity, respectively. This study investigated the potential effect of several variables on TDC of Bachelor of Science (BSc) students at an Australian research-intensive university. Results showed that two time-dependent variables (TDVs)--gender and Australian Tertiary Admission Rank (ATAR)--predicted TDC. Two time-independent variables (TIVs)--the number of discipline majors and specific year level academic achievement--also predicted TDC. Students who completed on time had a significantly higher mean weighted average mark (WAM) than non-TDC students, for each year of study, and more females than males completed their BSc in a timely manner. The primary determinants of TDC were gender, number of discipline majors, and WAM at each of years 2 and 3. Our conceptual model of TDC indicates the predictive interrelationships among these TIVs and TDVs. A more informed understanding of the study's outcomes among university stakeholders has considerable potential to enhance the engagement, scaffolding, achievement, and TDC of science undergraduates.