Guys trust me I have been working for Processors manufacturing companies for quite a long. Always remember to check your processor before going for any ram upgrades or downgrades. RAM bus speed totally depends only and only on the Processor you macbook has. For example, if you have late 2011 13 inch macbook pro 2.4 Cori5, then simply go to the processor specification page. It clearly shows that it supports 1066/1333 MHZ pc3 rams. Now based on that you can upgrade or downgrade without even having a single inch of doubt. However, if you try to do any other experiment where you want to put in a higher bus speed or a lower bus speed ram than specified, then there are 100% chances that it wont work. Take my tip - whenever deciding for a RAM, get exact specs of your processor and see the RAM compatibility list as RAM is designed to serve processor only and it should be considered before buying any RAM.