Treatment of ZSM-5 and Y zeolites with aqueous NH4BF4 (secondary synthesis) yields [B]ZSM-5 and [B]Y. Tetrahedral B3+ in the zeolite framework was verified by boron MAS NMR (B3+ shift : ZSM-5 = -3.6 ppm; Y = -2.7 ppm). B3+ chemical shift was found to correlate with the Si-O-Si bond angle (from silicon MAS NMR), further demonstrating that boron has been inserted in the framework. XRD and STEM showed increasing crystallinity losses with increasing amounts of framework B3+, and this is believed to be due to partial and random crystal collapse to accommodate the smaller B3+ ion. The NH4BF4 reaction is believed to insert B3+ via a metathesis pathway involving formation of various aluminum fluorides. XRD identified one product as (NH4)(3)AlF6, and aluminum MAS NMR (decoupling and saturation comb techniques) suggests a second product as [Al(H2O)(6)][AlF6]. This work demonstrates the versatility of the aqueous metal fluoride treatment for zeolite isomorphous substitution, particularly for previously difficult-to-prepare substituted zeolites.