It has been argued that the distinction between factivity and non-factivity is more fundamental to mental state understanding than that between false beliefs and reality. The present study examined children's growing understanding of all possible contrasts between the factive verb "know" and the non-factive verbs "think" and "guess" longitudinally between the ages of 50 and 94 months and investigated its relations to first- and second-order FB skills independently of general language skills. For this purpose, 140 monolingual children (63 females) were tested at 50, 60, 70, and 94 months in a task measuring their understanding of degrees of speaker (un)certainty expressed by the verbs "know," "think," and "guess" and in first- and second-order FB tasks. Results showed that by 60 months of age, "know" was differentiated from "think" and from "guess" in the majority of the sample. However, longitudinal results revealed protracted development over the preschool years. Further, there was a link between first-order FBU and a partial understanding of the factivity/non-factivity contrast at 70 months, while the consistent mastery of the know-contrasts at 94 months was predicted by second-order FBU. Results also suggested that false belief precedes a full-fledged concept of factivity rather than vice versa. The results are discussed in light of theories of socio-cognitive development.