The adjective ‘disappointed’ has to be followed by either the preposition ‘at’ or ‘by’.
However, I came across this sentence ‘used to show that you are disappointed about something’ at oxfordadvancedlearnersdictio … onary/pity.
Now I feel confused about the preposition that follows the adjective ‘disappointed’. If it can be used with ‘about’, then what’s the difference between ‘disappointed at’ and ‘disappointed about’? In what context do we use ‘disappointed about’?
Where did you get the first ‘rule’ you mention? Disappointed can be followed by other prepositions, for example ‘in’ or ‘over’.
Castle-hunters won’t be disappointed with the Isle of Man.
I was disappointed with the coverage on television.
I was disappointed by the lack of support.
You should have accepted that. I’m disappointed in you.
We are disappointed over the lack of funds.
Workers are disappointed about salary reforms at the factory.
Many of the prepositions are interchangeable in a variety of contexts.
I thought it should go like this as in ‘disappointed at/by something’ and disappointed in/with someone’?
IMO,
Disappointed about/at/by/in/of/over something; disappointed in someone.
You forgot with/about something, Haihao.
Leong - ‘with’ isn’t used with someone, unless there is a particular act that someone undertook involved:
“I am disappointed with the way in which you completed that.”
I saw this sentence ‘I was very disappointed with myself.’ at oxfordadvancedlearnersdictio … sappointed. Please enlighten me. Thanks. 
“disappointed with myself” is fine because “myself” is not really “someone”. Another example: normally you say: “I enjoy myself”, but rarely say: “I enjoy someone.” (should be under a very unordinary condition). A usage of “disappointed with” is such as:
Anila was a little disappointed with him for being so cliche.
Hope this can be of some help but not mislead you.
Hi,
I’d make a distinction between ‘disappointed in someone’ and ‘disappointed with someone.’
‘In someone’ suggests that your disappointment really derives from the person’s character/personality.
‘With someone’ suggests that your disappointment stems from what they have done.
Alan