Abstract: The goal of this study was to investigate, classify, and analyze the different types of writing errors committed by a select sample of Iranian university students participating in ESL programs. Furthermore, the study strived to discover whether or not the students and their instructors were aware of the mistakes that were made when the students were asked to write in two specific modes; argumentative and creative. One of the secondary objectives of this investigation was to classify the communicative value of errors. Such an evaluation was conducted with the help of three native speakers who used a 2 point scale to gage the level of hindrance that such errors had on the communicative ability of the students. The subjects participating in this study were 25 Iranian students who were all at the high-intermediate level in ESL programs at various universities throughout California. The data for this research was provided from a number of different sources. Such sources included the compositions written by the students, the results of interviews and consultations conducted with the subjects, and questionnaires that were provided to them to fill out. The findings of this study indicated that there was no fundamental significant difference between argumentative and creative essays as far the types of errors or the frequency of errors was considered, since the same types of errors occurred in both genres of writing. Additionally, the rank order of the different kinds of errors varied amongst the two modes of writing. The 828 total errors analyzed in this study were divided up among 10 major categories including errors with articles, prepositions, incorrect and confusing tenses, number conjunctions, adjectives, subjects and predicates, and verbs and pronouns. Within the scope of this study, it was impossible to empirically discover the causes of these errors. Therefore, the researcher assumes that the causes of such errors could be a result of the following: Interference of conversational English with written English, interference from Persian, the students' inadequate knowledge regarding certain structures, the sheer complexity of the English language, the transfer of training, lapses in memory, insufficient practice in formal writing, lack of familiarity with the rules regulating written English, and the overwhelming pressure placed upon students to focus their efforts and energy primarily on communication at the expense of grammar. Furthermore, it was realized that generally speaking, the most recurrently occurring errors were also the mistakes which placed the greatest hindrance on normal comprehension by native speakers. In this respect, it was concluded that the frequency with which an error occurred was not necessarily related to 'seriousness.'