The argument-adjunct distinction is highly discussed in the literature. There is an ongoing debate about the categorization of instruments. Instruments are categorized as arguments by some linguists, and adjuncts by others. This thesis revisits the argument-adjunct distinction and investigates the categorization of instruments in English and Turkish using both traditional argumenthood tests and reaction time studies. Phrases with instrumental case (Turkish -(y)le) or preposition marking (English with) can be clear arguments or adjuncts, but instruments seem to fall in between in both languages. Some argumenthood tests classify instruments as arguments and some as adjuncts. The current thesis thus adds support to previous studies that have argued that English instrument phrases have unclear status as arguments and adjuncts. Furthermore, the studies presented here found no statistical difference in reaction time categories between arguments, adjuncts and instruments. This might be because the specific method used was not sensitive enough.