When are 'tion', 'sion', and 'cion' used

Here are the main guidelines to choose the right suffix among -sion, -tion, and -cion. The first two are the more common while -cion is actually quite rare. (Lexico powered by Oxford)

Words ending in -sion

If the ending is pronounced as in confusion, then it should be spelled -sion. Here are some examples: collision; division; revision; persuasion; explosion; decision;seclusion.

When the ending comes after an -l, it's always spelled -sion: compulsion; revulsion; expulsion; emulsion; propulsion.

When the ending follows an -n or -r, it's often spelled -sion, especially if the word is related to one that ends in -d or -se. For example: immersion (from immerse); comprehension (from comprehend). Here are some more examples: aversion; conversion; apprehension; diversion; extension; version.

Nouns based on words that end in -ss or -mit always end in -sion: permission comes from permit and discussion comes from discuss Here are some more examples: commission; expression; aggression; admission; succession; impression; emission.

Words ending in -tion

If the ending is pronounced as in station, then it's spelled -tion. For example: addition; duration; nation; solution; ambition; edition; caution; position.

If the noun is related to a word ending in -ate, then the ending will be -ation, e.g. donation (from donate) or vacation (from vacate). Here are some more examples: accommodation; location; creation; rotation; education; mediation.

If the ending comes after any consonant apart from -l, -n, or -r, then the ending is spelled -tion: action; connection; reception; affection; interruption; description; collection; infection; deception.

After -n and -r, the ending can be -tion or -sion. It's more likely to be -tion if the word's related to another one that ends in -t or -tain, e.g. assertion (from assert) or retention (from retain). Here are some more examples: exertion; distortion; abstention; invention.

Words ending in -cion

There are just two common nouns that end in -cion: suspicion and coercion.