In this article, we explore the mapping and boundedness properties of linear and bilinear fractional integral operators acting on Lebesgue spaces with large indices. The prototype ν A0;-order fractional integral operator is the Riesz potential 96&_mathId=si1.gif&_user=111111111&_pii=S0022123616303196&_rdoc=1&_issn=00221236&md5=8e07cf170075e093857e2fac110ee52e" title="Click to view the MathML source">Iν, and the standard estimates for 96&_mathId=si1.gif&_user=111111111&_pii=S0022123616303196&_rdoc=1&_issn=00221236&md5=8e07cf170075e093857e2fac110ee52e" title="Click to view the MathML source">Iν are from 96&_mathId=si2.gif&_user=111111111&_pii=S0022123616303196&_rdoc=1&_issn=00221236&md5=75faae3c94ea2f4c169ab8b9d78be339" title="Click to view the MathML source">Lp into 96&_mathId=si3.gif&_user=111111111&_pii=S0022123616303196&_rdoc=1&_issn=00221236&md5=f3cdd959a191d1f5b61df2383d595453" title="Click to view the MathML source">Lq when 96&_mathId=si4.gif&_user=111111111&_pii=S0022123616303196&_rdoc=1&_issn=00221236&md5=e9e6c000b85fd9bd767a409f221053a0">96-si4.gif"> and 96&_mathId=si32.gif&_user=111111111&_pii=S0022123616303196&_rdoc=1&_issn=00221236&md5=5965d40a472d112f022525becde64ccb">96-si32.gif">. We show that a ν -order linear fractional integral operator can be continuously extended to a bounded operator from 96&_mathId=si2.gif&_user=111111111&_pii=S0022123616303196&_rdoc=1&_issn=00221236&md5=75faae3c94ea2f4c169ab8b9d78be339" title="Click to view the MathML source">Lp into the Sobolev-BMO space 96&_mathId=si54.gif&_user=111111111&_pii=S0022123616303196&_rdoc=1&_issn=00221236&md5=b95ab6851a303e49a0aa16b7b09c57ed" title="Click to view the MathML source">Is(BMO) when 96&_mathId=si7.gif&_user=111111111&_pii=S0022123616303196&_rdoc=1&_issn=00221236&md5=f8d572a69a02d03fe733edcb49a579b0">96-si7.gif"> and 96&_mathId=si8.gif&_user=111111111&_pii=S0022123616303196&_rdoc=1&_issn=00221236&md5=f312c8b3339a849ec7e3d09273afcf06" title="Click to view the MathML source">0≤s<ν satisfy 96&_mathId=si58.gif&_user=111111111&_pii=S0022123616303196&_rdoc=1&_issn=00221236&md5=fc8182b771c8cf17c2eaa6320d3645a2">96-si58.gif">. Likewise, we prove estimates for ν -order bilinear fractional integral operators from 96&_mathId=si10.gif&_user=111111111&_pii=S0022123616303196&_rdoc=1&_issn=00221236&md5=bf874ff9081478b59b6451ec436a2ab6" title="Click to view the MathML source">Lp1×Lp2 into 96&_mathId=si54.gif&_user=111111111&_pii=S0022123616303196&_rdoc=1&_issn=00221236&md5=b95ab6851a303e49a0aa16b7b09c57ed" title="Click to view the MathML source">Is(BMO) for various ranges of the indices 96&_mathId=si11.gif&_user=111111111&_pii=S0022123616303196&_rdoc=1&_issn=00221236&md5=002fae8aa964dabde87c643f0155f61d" title="Click to view the MathML source">p1, 96&_mathId=si12.gif&_user=111111111&_pii=S0022123616303196&_rdoc=1&_issn=00221236&md5=c530fad0e79aa63a6786fd8d9bad09b6" title="Click to view the MathML source">p2, and s satisfying 96&_mathId=si103.gif&_user=111111111&_pii=S0022123616303196&_rdoc=1&_issn=00221236&md5=4693c118743e88503574a05eb50fa113">96-si103.gif">.