Details between image 5 and 6 on this link
http://mr-ives.blogspot.ie/2011/03/replacing-mr16-halogen-bulbs-with-led.html[/QUOTE]
Rather surprised to find DC power supply all mine are AC and I would have expected the MR 16 lamp to have a rectifier inside the housing together with some control components as LED's use around 1.2 to 3 volt not 12 volt.
Running lamps designed for AC on DC could overload components within the lamp so I pulled out a new lamp to look at if AC or DC and there is nothing on the packet saying which it is.
With a LED the long lead is positive but with MR 16 both same length. I would not expect there to be a problem swapping the polarity and with a standard LED wrong way around it just does not work and it does not cause any damage.
I would not expect the rectifier to be damaged feeding DC rather than AC but it could in theory overload the rectifier one diode being unused and the other taking twice the power.
RS Components do seem to give more information than other suppliers and their bulb was listed as "Voltage Rating 9 → 15 V ac/V dc" so clearly could be AC or DC supplied.
The surprise was "for indoor use only" and must be used in a well ventilated area not totally enclosed.