两者意思差不多，但用法不同：upon 是介词，后接名词或动名词，而 as soon as 是连词，后接句子。比较：

1. upon(也可用 on)的例句

Upon arrival, they went in search of a hotel. 在到达时他们就去找旅馆。

Guests receive dinner on arriving at the hotel. 客人一到宾馆就可以用餐。

2. as soon as 的例句

They set to work as soon as they arrived. 他们一到，就开始工作。

As soon as she arrived she set about tidying up the room. 她一到，就开始收拾房间。

比较同义句(注意主句与从句主语相同)：

They set to work as soon as they arrived. 他们一到，就开始工作。

=They set to work on / uoon arrival. 他们一到，就开始工作。

As soon as she arrived she set about tidying up the room. 她一到，就开始收拾房间。

=Upon / on arriving she set about tidying up the room. 她一到，就开始收拾房间。