If you use it for crystallization, i.e. repeat from hits of the screen, you should use MES hydrate. This is how Hampton Research makes the buffer. You should be able to purchase the buffer from Hampton Research, or get some information form their website.

It depends on what you are using it for and if salt is an issue. The most formally correct way is to make a solution of both at the concentration you want (eg 100 mM) and then to add one into the other until you get the pH you want (monitoring with a pH probe). You can calculate the amount of each you need based on the dissociation constant. For MES it is about 44%acid to 56% salt to get pH 6.5 using this tool: http://www.currentprotocols.com/WileyCDA/CurPro3Tool/toolId-3.html.

Alternatively make up a concentrated stock solution of one and add acid or base until you get the correct pH then dilute to your working concentration and check the pH again.

Please se the following link:

https://www.goldbio.com/documents/3550/MES+Buffer+0.5M+Stock+Solution+with+table.pdf

Best regards

Vit