Pact of Research in Civilization V

Question

What are the requirements to have the option to sign a Pact of Research with another Civ?

Answer 1

Both sides need enough money for the agreement, and at least one side has researched Philosophy.

The cost for a research agreement scales with the era of the more advanced player and with the game speed:

Era         Quick  Standard  Epic  Marathon
-------------------------------------------
Classical   134    200       300   600
Medieval    167    250       375   750
Rennaisance 167    250       375   750
Industrial  201    300       450   900
Modern      234    350       525   1050
Future      234    350       525   1050

You can also have only one active research agreement per civilization, and research agreements are not possible during war (as bwarner pointed out).

If the enemy civilization has not enough money for the research agreement you can also gift (or loan it against gold per turn) the necessary gold to them.

Answer 2

To enter a research agreement, both parties need to have the required amount of gold and both need to have researched Philosophy.