Caffeine-treated wood is valued for its decay resistance, but caffeine high leachability under outdoor conditions significantly reduces its long-term effectiveness. This study aimed to mitigate the leachability of caffeine-impregnated Scots pine wood by incorporating either formaldehyde or glutaraldehyde. The treatments were evaluated using standard leachability analysis (EN 84), decay resistance testing (EN 113), mass spectrometry, attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy, and mechanical testing. Leaching tests revealed that caffeine-glutaraldehyde-impregnated wood exhibited the lowest mass loss after leaching (3.30 ± 0.15%), outperforming caffeine-treated (6.18 ± 0.15%) and caffeine-formaldehyde-treated (5.94 ± 0.13%) specimens, indicating superior caffeine fixation. ATR-FTIR spectroscopy showed that characteristic caffeine bands diminished following leaching, confirming caffeine leachability. However, caffeine-glutaraldehyde-treated samples exhibited less pronounced spectral changes compared to caffeine-formaldehyde-treated samples. Mass spectrometry further corroborated these findings, detecting higher caffeine content in glutaraldehyde-treated wood than in caffeine-only specimens after leaching test. Decay resistance tests demonstrated that caffeine-glutaraldehyde-treated wood retained high resistance to fungal decay both before and after leaching. Mechanical tests revealed that the modulus of rupture (MOR) was preserved post-leaching only in caffeine-glutaraldehyde-treated samples. These findings highlight the effectiveness of low concentrations of glutaraldehyde in reducing caffeine leachability, thereby enhancing the decay resistance and durability of treated wood, making it a promising approach for outdoor applications.