In this study, we investigate the processing of object-denoting emoji in sentences using eye tracking. We hypothesize that (a) such emoji are more difficult to process when used as word replacement; and (b) their processing is subject to ambiguity constraints similarly to what happens with words. We conduct two experiments in which participants have to read sentences in which an emoji either follows or replaces a word. Control stimuli not containing emoji are also tested. In the second experiment, the emoji are presented in two different disambiguating contexts. First fixation duration, total visit duration, and total revisit duration are modeled in the various conditions using linear mixed models. Both our hypotheses are supported. We observe longer total visit time for non-redundant emoji, and higher values for all three measures for ambiguous emoji. We conclude that lexical access may be more difficult for emoji especially when they are used as word replacements and their meaning is not immediately clear. Furthermore, we also conclude that non-redundant emoji are more difficult to integrate in the processing of the sentence than the equivalent words, or emoji used in a redundant way. In turn, our results indicate that emoji may not always be as immediate and easy to process for readers in spite of their popularity.