In this paper interlanguage development of two adult learners is observed during a one year period. The theoretical section of the paper provides a description of the concept of interlanguage while simultaneously showing the development of the interlanguage hypothesis. The second chapter deals with the description of the Processability Theory which was used for the corpus analysis and the third chapter explains the age factors in language acquisition considering that the research subjects are adult learners of English as a foreign language. The research part of this paper provides an analysis of tests for written and oral production, which were used to monitor interlanguage development of two adult learners. In order to analyze the interlanguage, it was necessary to describe the developmental stages for acquiring the structures in question. Starting from the assumption that developmental stages cannot be skipped, during the acquisition of the structures in question six developmental stages were identified. Corpus was analyzed using the emergence criterion which is defined as the beginning in the process of the acquisition of a certain structure. The first step in emergence analysis is the distributional analysis which shows mapping of a certain form on a certain function, that is, the dynamic nature of interlanguage. The second step in corpus analysis was the separation of systematic productivity from the patterns. Productivity was measured with the number of tokens and the systematic use of lexical/morphological variations of those tokens. Finally, implication scaling was done, which determined that developmental stages cannot be skipped. In order to show not only emergence, but also the acquisition of a certain developmental stage, Brown’s criterion was used, according to which a certain structure is considered acquired if it is used with 90% accuracy. Based on the results of the analysis it can be concluded that during written testing both research subjects demonstrated structures matching developmental stages 3, 4 and 5, even though only developmental stage 3 can be considered acquired. During oral testing structures matching developmental stages 2, 3, 4 and 5 emerged, and also only developmental stage 3 can be considered acquired. Research subject Vera was more successful in written tests and during oral testing research subject Slaven achieved much better results. When comparing oral and written tests, both research subjects were much more successful during the latter. It was determined that the structures were acquired according to the predicted order and that interlanguage developed in a similar manner for both research subjects, that is, after apparent progress there is regression and then finally there is progress again. This research therefore confirms the predictions presented in Processability Theory and expands the theory by applying it to written production. It also suggests the classification of subject questions to developmental stage 4 which represents a significant contribution of this paper.