Scholars have been dubious about about reconstructing i-stem adjectives for Proto-Indo-European because it is not possible to reconstruct many word equations among IE branches, but we have good evidence for the derivational processes that were used to create secondary i-stem adjectives already in the proto-language. The most productive derivational process for i-stem adjectives is the nominalization and readjectivalization of o-stems. The first step of this formation is the creation of i-stem nominalizations of o-stem adjectives, and the second step is the readjectivalization of the i-stem substantivization, e.g. Gr. argós 'bright’ vs. Gr. argi- 'white’, 'brightness’, Hitt. harki- 'white’.The other possible fashion of creating i-stem adjectives is by vrddhi derivation, genitivals formed with the insertion of an increased ablaut grade in the root, e.g. Gr. akro- 'topmost’ vs. Lat. ācer 'sharp’. I have collected simplex i-stem adjectives of several IE branches and performed a morphological and etymological analysis of each one of these adjectives. I have focused on the branches that have the most cases of i-stem adjectives, such as Celtic, Italic, and Anatolian; and I also discuss cases from IE branches with less cases, such as Vedic, Avestan, and Ancient Greek. In the Celtic branch, I focus on Old and Middle Irish, where i-stem adjectives are the third most productive class of adjectives, e.g. maith 'good’, áith 'sharp’. In the Italic branch, I focus more on Latin, where we can find many simplex i-stem adjectives, e.g. dulcis 'sweet’. In the Anatolian branch, I focus on Hittite, which has cases like nakkī- 'important’, and šalli- 'big’. Indo-Iranian and Greek have less i-stem adjectives, but we can still find cases like Ved. hári- 'golden yellow’ and Av. zairi- 'tawny’. My research shows that the process of the nominalization and readjectivalization of o-stems into i-stems is the most productive process to create secondary i-stem adjectives in Indo-European. Most languages, except for Greek, also preserve a few cases of i-vrddhis, a much rarer derivation process.