We study the determinants of private bene…ts of control in negotiated block transactions. We estimate the block pricing model in Burkart, Gromb, and Panunzi (2000) (BGP) acknowledging the presence of both block premia and block discounts in our sample. We …nd evidence in favor of the BGP model according to which the occurrence of block premia and block discounts depends on how competitive the block seller can be in opposing a potential tender o¤er for the target's stock. Private bene…ts represent 3% of the target …rm's stock market value. In addition, our approach allows us to measure the e¢ ciency with which private bene…ts are extracted: On average, each $1 of private bene…ts costs shareholders $2 of equity value. Private bene…ts decrease with target's size and short term debt, and increase with target's past performance, intangible assets, and cash. The later e¤ect is stronger if the target's cash is higher than the acquirer's cash. Acquirer's overpay an average of 7% of the target's stock price relative to the BGP benchmark. We use our structural estimation to conduct a counterfactual policy evaluation of the Mandatory Bid Rule. Our results suggest that the Mandatory Bid Rule fails to add value to shareholders because it fails to prevent welfare decreasing transactions and deters welfare increasing transactions by forcing ine¢ cient tender o¤ers.