Abstract – This corpus-based study examines the diachronic development of the that/zero alternation with nine verbs of cognition, viz. think, believe, feel, guess, imagine, know, realize, suppose and understand by means of a stepwise logistic regression analysis. The data comprised a total of (n=5,812) think, (n=3,056) believe, (n=1,273) feel, (n=1,885) guess, (n=2,225) imagine, (n=1,805) know, (n=1,244) realize, (n=2,836) suppose and (n=3,395) understand tokens from both spoken and written corpora from 1580–2012. Taking our cue from previous research suggesting that there has been a diachronic increase in the use of the zero complementizer form from Late Middle / Early Modern to Present-day English, we use a large set of parallel spoken and written diachronic data and a rigorous quantitative methodology to test this claim with the nine aforementioned verbs. In addition, we also investigate the impact of eleven structural features, which have been claimed to act as predictors for the use or presence of the zero complementizer form for ‘panchronic’ (i.e. effects are aggregated over all time periods) and diachronic effects. The objectives of this study are to examine the following: (i) whether there is indeed a diachronic trend towards more zero use; (ii) whether the conditioning factors proposed in the literature indeed predict the zero form; (iii) to what extent these factors interact; and (iv) whether the predictive power of the conditioning factors becomes stronger or weaker over time. The analysis shows that, contrary to the aforementioned belief that the zero form has been on the increase, there is in fact a steady decrease in zero use, but the extent of this decrease is not the same for all verbs. In addition, the analysis of interactions with verb type indicates differences between verbs in terms of the predictive power of the conditioning factors. Additional significant interactions emerged, notably with verb, mode (i.e. spoken or written data) and period. The interactions with period show that certain factors that are good predictors of the zero form overall lose predictive power over time.