This study investigates whether I’m sure seems to be on the same grammaticalisation trajectory as I think. It does so by tracking the frequency of these two constructions over time to explore (i) their distribution across clausal positions (syntagmatic variability) and (ii) the extent to which the complementiser that is omitted (paradigmatic variability). The study uses spoken data from the BNC and the newly compiled Spoken BNC2014. The results show that the two constructions exhibit remarkable similarity, not only in terms of their proportional distribution across clausal positions, but also in terms of their propensity for that-omission. For example, both constructions show adverb-like behaviour with regard to clausal positions. Furthermore, even though the time span covered is relatively short, a clear increase in that-omission was noted for I’m sure, mirroring the frequencies for I think very closely. It thus seems that I’m sure is on the same path as I think, despite differences in frequency entrenchment