A number of adjectives with the suffix -ive are derived from verbs. The verbs end with the sound /d/. This changed to /s/ as a result of spirantization.
Here is a list of adjectives formed from verbs:
The adjectives exhibit spirantization:
comprehend + ive = comprehensive
conclude + ive = conclusive
decide + ive = decisive
defend + ive = defensive
divide + ive = divisive
expand + ive = expansive
explode + ive = explosive
offend + ive = offensive
persuade + ive = persuasive
respond + ive = responsive
Many adjectives with the suffix -sive are derived from base verbs which have final syllable stress and end in /d/. The /d/ later changed to /s/, a sound change known as spirantization. Spirantization is a type of palatalization, one in which a plosive changes into a fricative before a high vowel. This sound change is one of many in English.