SemGroup ® Corporation (NYSE: SEMG) (“SemGroup” or the “Company”) today announced that its Board of Directors has unanimously adopted a limited duration Stockholder Rights Plan (the “Rights Plan”) and declared a dividend of one right on each outstanding share of SemGroup’s common stock. The Rights Plan was adopted in response to the unsolicited proposal announced by Plains All American Pipeline, L.P. to acquire all of the outstanding shares of the Company for $24.00 per share in cash which, in the unanimous view of the entire Board of Directors, substantially undervalues the Company. The Rights Plan is designed to assure that all of SemGroup’s stockholders receive fair and equal treatment in the event of any proposed takeover of the Company and to guard against tactics to gain control of SemGroup without paying all stockholders an appropriate premium for that control. The Rights Plan is intended to enable all SemGroup stockholders to realize the long-term value of their investment in the company. It will not prevent a takeover, but should encourage anyone seeking to acquire the Company to negotiate with the Board of Directors prior to attempting a takeover. Pursuant to the Rights Plan, SemGroup is issuing one right to purchase one half of one share of common stock for each current share of common stock outstanding at the close of business on November 7, 2011. Initially, these rights will not be exercisable and will trade with the shares of SemGroup’s common stock. Under the Rights Plan, the rights generally will become exercisable only if a person or group acquires beneficial ownership of 10% or more of SemGroup’s common stock in a transaction not approved by SemGroup’s Board of Directors. In that situation, each holder of a right (other than the acquiring person, whose rights will become void and will not be exercisable) will be entitled to purchase, at the then-current exercise price, additional shares of common stock having a value of twice the exercise price of the right. In addition, if SemGroup is acquired in a merger or other business combination after an unapproved party acquires more than 10% of SemGroup’s common stock, each holder of the right would then be entitled to purchase at the then-current exercise price, shares of the acquiring company’s stock, having a value of twice the exercise price of the right.