  This paper studies weakly mixing (singular) and mixing masas in type
$\rm{II}_{1}$ factors from a bimodule point of view. Several necessary and
sufficient conditions to characterize the normalizing algebra of a masa are
presented. We also study the structure of mixing inclusions, with special
attention paid to masas of product class. A recent result of Jolissaint and
Stalder concerning mixing masas arising out of inclusions of groups is
revisited. One consequence of our structural results rules out the existence of
certain Koopman-realizable measures, arising from semidirect products, which
are absolutely continuous but not Lebesgue. We also show that there exist
uncountably many pairwise non--conjugate mixing masas in the free group factors
each with Puk\'{a}nszky invariant $\{1,\infty\}$.
