NOVELTY - Synthesizing a polyacetal (I) by the reaction of an aldehyde and/or ketone on a light polyacetal (II), using a simulated moving-bed reactor (SMBR) in which the columns are filled with a solid acid susceptible to adsorb selectively at least one of the reaction products, comprises: introducing reagents and eluent; catalytic reaction to produce the target polyacetal; separating the residual reagents and the products by selective adsorption in the reactor; and extraction of polyacetal solution and various reaction byproducts in mixture with the eluent at two distinct points of the SMBR.    USE - The process is useful for synthesizing polyacetal i.e. useful as constituents or diesel fuel additives or fuel for fuel cells e.g. direct methanol fuel cells in methanol substitution. The process is useful for continuous industrial manufacture of polyacetal.    ADVANTAGE - The process provides an improved and high selectivity for conversion in simulated moving-bed, and very fast polycondensation reaction and almost immediate separation.    DETAILED DESCRIPTION - Synthesizing a polyacetal of formula R1-(O-CR2R3)p-(O-CR4R5)m-O-R6 (I) by the reaction of an aldehyde and/or ketone on a light polyacetal of formula R1(OCR2R3)pOR6 (II), using a simulated moving-bed reactor (SMBR) in which the columns are filled with a solid acid susceptible to adsorb selectively at least one of the reaction products, comprises: introducing reagents and eluent; catalytic reaction to produce the target polyacetal; separating the residual reagents and the products by selective adsorption in the reactor; and extraction of polyacetal solution and various reaction byproducts in mixture with the eluent at two distinct points of the SMBR.    R1, R6 = (cyclo)alkyl or 1-20C aryl;    R2-R5 = 0-8C hydrocarbon, preferably 0-4C atoms;    p = greater than or equal to 1; and    m = 1-8.    Provided that the total number of carbon atoms in the molecule, i.e. R1+p(R2+R3+1)+m(R4+R5+1)+R6 is less than or equal to 20 and preferably less than or equal to 12.