surprised

Surprised can be followed by a verb (to + infintive), or by a noun - surprised at + noun, in your example you use the gerund seeing which is really just a verb in a noun form. You could also say surprised at the weather, surprised at the result, etc.

Hi sidewalker,

Thank you for your reply. What is the difference in meaning between them?

For your two examples there is no difference in meaning.

Just a note - "at" is a preposition - which is why it must be followed by a noun phrase or gerund. So as Sidewalker says there's no difference in meaning, just a difference in the construction. Sometimes the verb is actuallytredundant, giving the option of all three :
I was surprised to hear the news
I was surprised at hearing the news
I was surprised at the news

- but there might be times when there is no possible verb construction - eg :

I was surprised at the weather. I didn't expect it to be so cold in July.
I was surprised at the price. I had expected it to be much more expensive.

so that the preposition + noun phrase construction is the only one possible.

"at" is also often followed by "wh" clause - especially begining with "how": Compare :

I was surprised at his words.
I was surprised at what he said

I was surprised at the weather
I was surprised at how cold it was.

I was surprised at the cost,
i was surprised at how cheap it was.

fface wrote:

I'm surprised to see you here? (from a dictionary)
I'm surprised at seeing you here?

Hi susan,

Which kind of construction is more common and why? surprised to see...or surprised at seeing...?

Thanks a lot.

to + infinitive

Using a corpus of 3m words, both US and UK English, I got the following results :

surprised to + infinitive : 16 occurrences
surprised at + gerund : 0 occurrences

surprised at plus other form of noun phrase eg with noun head or pronoun) : 18 occurrences