The present invention is related to the use of a new conditioning agent for treating pelts for tanning. This conditioning agent consists in a composition of modified aldehydes, carboxylic acids or polycarboxylic acids and additives. The conditioning agent is used for replacing the steps of deliming and pickling in the pelts treatment. The action of the product can eliminate the step of basification when the tanning is made with chromium salts. The objective of the present invention is the use of a novel composition that can replace the steps of deliming, step 6, and pickling, step 8, described before for one just step called conditioning The proposed system, neutralizes the alkalinity and hence removes the solubility of Calcium from the liming operation attached in the pelt. The system described is obtained from functional aldehydes with the general formulation as following: The system comprises functional aldehydes to which are added a combination or just one of the following chemicals: mono, di, tri and polycarboxilic acids; anhydrides: fumaric, maleic and ftalic anhydrides of dicarboxilic acids that contains at least 2 to 8 carbon atoms; esters of mono, di, tri and polycarboxilic acids; EDTA; boric acid and sodium and calcium tripolyphosphates As solvents to stabilize the system it is used one or a mixture of the following compounds: water, branched or non-branched aliphatic alcohol, glycols and polyols.