Solid forms comprising (+)-2-[1-(3-Ethoxy-4-methoxyphenyl)-2- methylsulfonylethyl]-4-acetylaminoisoindoline-1,3-dione, compositions comprising the solid forms, methods of making the solid forms and methods of their use are disclosed. The methods include methods of treating and/or preventing disorders ameliorated by the reduction of levels of TNF-± or the inhibition of PDE4.