e.g., $C{H_3} - O - {C_2}{H_5}\, + \,HI\,\xrightarrow{{373\,\,K}}C{H_3}I\, + \,{C_2}{H_5}OH$

The alkyl halide is formed from the smaller alkyl group.

However, in case of tertiary alkyl ether following reaction occurs.

$\mathop {C{H_3} - OC{{(C{H_3})}_3}}\limits_{ter - butyl\,\,methyl\,\,ether}  + \,HI\,\xrightarrow{{373\,\,K}}$  ${(C{H_3})_3}C - I\, + \,C{H_3}OH$

The alkyl halide is formed from the tertiary alkyl group and the cleavage of such ethers occurs by $S_{N^1}$ mechanism as the product is controlled by the formation of more stable intermediate tertiary carbocation from orotonated ether.