Law enforcement sources tell TMZ … T.I. and his wife Tameka “Tiny” Cottle were arrested because cops claim they found what appeared to be ecstasy in their car.

T.I. and “Tiny” were arrested on the Sunset Strip last night after making an illegal U-turn.  Cops say they smelled marijuana, conducted a search and found a controlled substance.

They were told that the substance was allegedly ecstasy.  And, their law enforcement sources call it a “small amount.”

The pills are being tested.

TMZ also believes that  the styrofoam cups in the front and back cupholders contained Sizzurp

 — A form of codeine falls under the class of drugs for which T.I. and his wife, Tameka “Tiny” Cottle, were arrested…. although cops aren’t saying which controlled substance they found in the car, there seems to be evidence of Sizzurp … aka purple drank … aka codeine syrup.

Cops won’t say specifically which drug they found, but TMZ’s sources say it’s the class of drugs that does indeed include the main ingredient for Sizzurp.

Both T.I. and “Tiny” were booked on felony possession of a controlled substance.