Research on several Indo-European languages attests to notable difficulties in inflecting irregular nouns and verbs. In these languages morphological and phonological factors are often intertwined in a way that obscures the source of the problem. Hebrew by contrast allows isolation of morphological and phonological factors in nominal inflection. Three experiments demonstrated that as in Indo-European languages, nominal inflection of Hebrew irregular nouns is slower than that of regular nouns and involves more errors. The occurrence of phonological alterations to the noun’s stem with the inflection is an additional source of irregularity, which also taxes the inflectional process in reaction time and error rate. The empirical results underline the power of the default automatic suffixation process as the main obstacle to irregular inflection. A theoretical contribution of this study is an interpretation of the irregularity effect based on a morphological analysis that views Hebrew as having a linear rather than a non-linear morphology. The stem–suffix match is suggested as the dominant factor affecting the inflectional process, responsible for the difficulties in irregular inflections. It is argued that in Hebrew, the differences between inflecting regular and irregular nouns can be easily and adequately explained as resulting from a mismatch between a stem and an affix.