• If the ending is pronounced as in confusion, then it should be spelled -sion. Here are some examples:
  

• When the ending comes after an -l, it's always spelled -sion:

• 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) and comprehension (from comprehend). Here are some more examples:

• Nouns based on words that end in -ss or -mit always end in -sion. For example: permission (from permit) and discussion (from discuss). Here are some more examples:

Words ending in -tion

• If the ending is pronounced as in station, then it's spelled -tion. For example:

• If the noun is related to a word ending in -ate, then the ending will be -ation. For example: donation (from donate) or vacation (from vacate). Here are some more examples:

• If the ending comes after any consonant apart from -l, -n, or -r, then the ending is spelled -tion:

• 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. For example: assertion (from assert) or retention (from retain). Here are some more examples:

Words ending in -cion